I just un-jailbroke my iPhone 3G after being jailbroken since August of 2009.
I jailbroke originally for 3 reasons: Cycorder (video recording), GV Mobile (for Google Voice), and Backgrounder (to run apps in the background).
Once Qik was released in the app store, and I learned VoiceCentral was being re-released as a web app soon, and since the iPhone 3G doesn't really work well with multiple apps running, I decided to go back to regular firmware.
I am glad I did. I had been suffering from self-assigned IP addresses on all of my Macs and iPhones for months and I didn't know why. Not until I un-jailbroke my phone did I put two and two together and realize that the iPhone was the problem.
There are a few things I miss. I miss the 5 icon dock, SBSettings, and Status Notifer, but none of those things are worth the pain of the self assigned IP Address problem I lived with for 6 months.
I also had no desire at all to use any themes, so that wasn't an issue for me.
If you use Apple routers like I do, I recommend against it because of the self assigned IP address problem. If you don't, or you want to see if you don't get that problem, then by all means give it a shot. It is very easy to do and there are several good utilities in Cydia. But you should know, that just like the app store, the majority of apps in Cydia are crap as well.