What's the big deal with foursquare?

So you've probably seen the name Foursquare thrown around in conversation a bit lately.
But even having a quick look at the site doesn't help you get a good understanding.
What is this damn thing that everyone keeps talking about?

Basic Rundown: It is an application that you can use from your mobile phone or PC. It uses real locations to which you check-in when you actually go there.
There are a few different reasons you might want to use such an application. Fun, Social Events, Calendar/reference, Shopping.

It might pay if I go into a bit more detail here.

Game/Just-For-Fun.
Each time you check-in to a location you are given points.
1 point for your first stop of the day.
2 points for your second stop of the day and so on.
There are also bonus points. You will be awarded a bonus 5 points the first time you check-in to any location and a further 5 points if you add a new location.
With this there is a leaderboard for your local city this resets every Sunday night ready for the new week. So you don't win anything for being #1 in your city, It is a great excuse to go out and socialise.

If you check-in to a location you are able to see anyone else who is currently checked in as well as a map.
Once you check-in to a location more than anyone else you become the mayor. There can be privileges for being mayor. See the next topic "shopping"
There is also the ability to earn badges. There are many badges available for all different things: Your first check-in, being Mayor of 10 places at once, Going out on consecutive nights. etc.

Shopping.
At each location you have the ability to add tips. If you see a special why not share it for others? If anyone checks in nearby they will get a message alerting to them to that deal.
You can also keep track of all of your favorite shops.

There are also businesses who use foursquare to their advantage by offering certain deals to whomever is currently Mayor. One example of this is Greystone Bar & Cellar in Brisbane Their promo is Whoever is Mayor on the first Friday of the Month gets a cocktail on the house!
There is a full list of businesses who have promo offers. However this is still pretty new so there aren't too many places listed locally yet.
Got a shop you love? why not suggest they get on-board and run promos? It can only work to their advantage.

If you look at a venue online you are able to see more information than you can from your mobile phone. Read what other people have to say about the place before you spend your money.

Calendar/reference
If you get into the habit of checking in everywhere you go the online history can be really handy. For example imagine you borrowed some books from the library but lost the little slip they gave you with the due date. Just check out your history and you can see when you last went to the library.
I've used this service a number of times for reference. I am a member of a just-for-fun fitness group. we track our distances each week and then submit them the goal being to see which team can make the virtual trek across Australia first. I'll be honest I am shocking at remembering to write my distances down. Once a week I log in to my foursquare history and can easily work out where I walked through the week and can work out my distances from there.
Another use I found for this service came a couple of weeks ago.
I went to the fridge to grab some carrots I had bought a few days prior. They were soft and unuseable. As I shop around most of the time for bargains I'm not always sure where I have bought my individual produce. I checked my history, found I had bought them only 3 days before which is not good enough in my books. I now know not to buy my carrots from this place anymore.

Tips & hints.
Don't forget this is a social networking site so any information you post may be visible to others. If you are cautious of protecting your identity I suggest you use a username rather than your real name. I also suggest that if you plan on adding your own house or place of work it may pay to just add a general address instead of the exact whereabouts. There is also a function where you can stay "off the grid" This is when you turn off the "Tell friends" function.
You can link your foursquare account to facebook and twitter if you want. That way if you are going to a big social event you can let the world know. but that is purely up to you. I do warn however if you broadcast your every whereabouts you leave yourself open for creepy stalkers or run the risk of aggravating your friends/followers. I suggest you use the auto-post function in moderation.
One lesson I did learn... Do not grant access to twitter and then revoke the access if you want to later allow again. There is a bug in the system which triggers and you are no longer able to get in and just grant access again. You have to ask the guys from foursquare to manually fix it for you. Learn from my mistake.

If you find some other use I haven't mentioned. Drop me a comment. I'd love to hear it.