To be honest if you are happy with your phone the way it is then dont bother jailbreaking it.
If you want a few nice mods/tweaks to be able to put on your device then do it. Many of the new things that Apple put into their iOS4 were available via Jailbreak before, and many of the things they will put on iOS5 are already available now
Obvious one is angry birds but you should also get shazam which is handy, if you dont what song your listening to you open the app let it listen and it tells you, brilliant. Btw Iphone 4 is the best choice you can make, i love mine