I got a Galaxy S, I update it frequently with the last Android version. Actually, i use the 4.1 version with the SlimBean 3.0 ROM.
SlimBean is a very light and fast ROM, i like it.
I only use root for the theathering, because it's bypass the limitation of my operator. (Theathring is a expensive option, but in root mode is free

)
If you are a old rom (Without semaphore recovery mode), it's could be a little difficult to update it. But as soons as Semphore Recovery is installed, you could install ROMs & Apps automaticaly and easily.
With some ROMs, you could experience some bug, but frequently updates are useful to fix them. My original branded ROMs has worked very bad with the bluetooth of my car, but with SlimBeam it's works perfectly.