personally ive had my iphone jailbroken for all the time i have had it, and its been very good because i have been able to have 3rd party applications not apple approved and i can ssh int my iphone and change my ringtones to mp3s and much more, it does not harm the phone at all..
you can change your boot and recovery logo lol ive changed mine to a apple logo with a penguine inside
and the sliider to unlock to a bullet, you can pretty much change everything and apple seem to not like this, but the head guy of apple steve whats his face even he jailbrkeas his iphone!!! with pwnage tool of course
pwnage is for mac users, if your pc like me then its winpwn....
apple updates... no if you dont have problems now with apple update then you wont after jailbreaking it either, it just means when you update your phone will become un-jailbroken and you can either jailbreak again with new software or not bother depends what you use your iphone for and if its worth it to you... some people are more then happy with app store and dont want to jailbreak because they are legit. and others jailbreak everytime they upgrade.
2.1 beta is out now firmware so later this month there will be the official 2.1 release
because my iphone is not legit i have to wait for a unlock if i want to upgrade.