I ran my 3gs jailbroken for awhile, off & on, but I always ran into more memory issues.
Then there’d be an update & I’d go back to “stock” for awhile.
Honestly, I’d wait if I were you. iPhone 4.0 OS will be released in June, and it’s going to be sweet. I’m running the beta 1 now and I love the folders & multitasking.