I've been using Foursquare, Gowalla, Sekai Camera location-based services.  They all have their pros and cons, but none of them is the clear winner, yet.  Find out how they compare and which one you may want to start with.

First of all, I should point out that I've used Foursquare the most and Sekai Camera the least.  I've been using them in parallel but I won't do that for long.  At some point, I'll pick one and put the others in the shelf.

Bottom line, for me it is still a horse race between Foursquare and Gowalla.  Yelp just doesn't raise my pulse, I give it a big meh for now.  Sekai Camera is in a different class.

As it is right now, it doesn't have much of a chance against the others mentioned here, but as mobile phones get more powerful, it or something very much like it will be a big, big hit.

Dream or Nightmare?

I love the idea of being able to look back and track where I've been and when.  I realize that this may be a nightmare to others, but to me these are more breadcrumbs in my digital trail.

From my point of view, the more details I can delegate to my digital persona to remember, the more I can focus on more creative acts, complex problem solving and, generally, the things humans are really good at and get a lot of satisfaction from.

Picture an old movie where the absent-minded professor accomplishes amazing acts of discovery and wonders, thanks in large part to his quiet companion who lays down the planks under the professor's feet as he is about to wade into a river, just to get to the other side.  That's what I want my digital persona to do for me one day.

From that perspective, location-based service, social networking, etc are but experiments along the way.  In the aggregate, we are making a huge investment of time and money and sheer brain power that will get us there sooner rather than later.  For that I am thankful and I am just doing my small part to move it along.

Foursquare

Usage:

• I have the most "friends" in Foursquare, so I get more notifications from it than any of the others.
• This is the first app I go for when I get to a place I want to check-in at.

PROs:

• Nice UI, well designed for its primary job: checking in.  If you're lucky, you can do it in three clicks.
• They finally got rid of the "cities" concepts.  It was a big obstacles for new users (including me). 
• Call me silly, but I really like the "Mayorships" and the badges.  When I started using it, I found myself wanting to go to different places to get more badges (and, no, I've never been a BoyScout, so it's not a "repressed emotion" thing).

CONs:

• Most of the time, I am not lucky and it takes more than three clicks.  Sometimes a whole lot more.  That's when the place I am does not show up in the list of nearby places even though I've been there before.
• Some places don't show up at all in the list and I have to search for them.  For example, I am the Mayor of a particular Bank of America branch.  I created the place and seem to be the only person who cares to check-in at it.  Nevertheless, this location never shows up in the list of nearby places.  I have to find it by running a search.
• The service seems to be overloaded at times because sometimes it is really snappy and some times it just drags.

Gowalla

Usage:

• I don't have as many direct "friends" in this app but I see a lot more people that have "been there."  At my local Safeway, for example,I am still the Mayor in Foursquare even though I go there maybe once a week; in Gowalla, I see a ton of people who've been there.

PROs:

• Connectivity is consistently snappy.  Long lists (places, people) take a bit to load, but they always load in ye long, consistently.
• If the places has is already been created (and most of the places I go to are), you can check-in in four clicks, consistently.  The place I am at always shows up at or near the top and it also shows a big checkmark next to the ones I've checked-in at previously.

CONs:

• You have to be physically at or very close by the place you want to check-in at in order to check-in.  This is kind of deal killer for me (and for @scoblelizer, too). For example, the BoA branch I mentioned above, is across the street from a Starbucks I frequent.  Sometimes, I forget to check in at the bank until I get to the Starbucks.  No problem doing that with Foursquare, but Gowalla won't let me.  FAIL!
• You cannot even see who's at a certain place unless you're checked-in at that place.  For example, if I see @friendA at a location, and I know that @friendB also frequents that location, I may look to see if I can introduce them to each other so that they can meet in person and go on to create the next Google.  But, nooooooo, that could never happen using Gowalla.
• The whole concept of dropping and picking up items is bogus and does nothing for me.  If they want to create a bit of drama and competition, I think Foursquare has a more intuitive model for that (the Mayor thing took a bit of getting used to, but who doesn't understand badges?).

Yelp

Usage:

• I have no "friends" in Yelp.  Prior to its new check-in feature,I've used as a Yellow Pages directory.  I've never cared much for restaurant reviews from strangers (your road-to-ecstasy dish is my gag-inducing nightmare and viceversa).  I find the Foursquare "tips"are more useful ("try the fried bananas--they're sweet and delicious").

PROs:

• It seems that every place I go, it's already there, at least for the Bay Area.  This means that I can check-in in three clicks, every time.  (We'll see how it goes with creating a new place.)

CONs:

• The feature seems bolted on.  There's no drama to it.  This is not a deal killer, but I can't see it generating a whole lot of enthusiasm for it, except maybe for long-term users of Yelp.  They probably did the right thing to get it out quickly and add the bells and whistles later, but still…
• I ain't got no friends, but that's my bad, not theirs.  Or it may be for the same reason that I never developed a group of friends in MySpace: it was just not the place for the type of people who happen to befriend me and viceversa.  That's something for Yelp to think about.

Sekai Camera

Usage:

• I have only a couple of "friends" and they are not people I run into or converse with very often.  In fact, one of them I have not interacted with since we became friends.
• I don't use consistently everywhere I go as I do the other three apps.
• I am using it with an iPhone 3G and it seems to be meant for a 3GS.  I am just grateful that I can play with it at all with my phone.  Yelp was rumored to do a similar thing but I've never been able to make it work with my 3G.

PROs:

• It is immensely fun.  At least, the idea of it is.

CONs:

• In practice, I have not found many places with air tags in it (except where I and my friends have left them).
• It sucks up the battery like there's no tomorrow.
• Connectivity to the server is slow at best and it breaks the flow of usage.

In Closing…

One scenario is that Foursquare ends up on top by learning and moving faster than Gowalla and other competitors.  Getting rid of the "city" concept took guts and I imagine a bit of architectural rework.  They listened, got the message and acted.

They may even end up ahead of Yelp if the latter continues to defend its original space and treats these "upstarts" as fads.  It sure took them a long time to come out with a feature that Foursquare quickly demonstrated to be very popular and viral.

In another scenario, Yelp ends up on top by leveraging the presence and audience they already have and muscling its way to success.

Of course, the third scenario is that Foursquare continues to build up steam, unabated and then Yelps acquires them (i.e., think Google and YouTube or Facebook and FriendFeed).

No matter if any of these or another scenario comes to pass.  In the end, these location-based services will become more prevalent and will show up in more and more apps.  So, get used to the idea that your digital trail will have more and more breadcrumbs in it in the future.  You won't be able to avoid it; you won't even want to avoid it in the end.

UPDATE: It seems that Foursquare is leaping ahead with Gowalla as a close second.  Regardless, for me, Foursquare seems to be it: the place where most of the people I want to hang out with are.  I still have the other clients installed in my iPhone, but I check-in only with Foursquare.

UPDATE: Foursquare and Gowalla released new version of their iPhone clients to coincide with the start of SXSW.  They are both prettier but buggier, particularly Foursquare's.  In fact, Foursquare released an update to the update three days later :)