If you decide to do yours, be sure to back it up first. Not sync. With it plugged in and iTunes running, on the left side right click the phone and choose back up. Now, after the update, if you don't like it you can restore it to the previous state.
I have an iPhone 3G and by chance, I haven't ever bothered updating the firmware from 3.1.3. to 4.2. The last time I considered it, I read that 4.2 was very slow for the 3G and made the phone unusable.Regardless, I was considering to update my phone anyways, and decided to ask you guys for advice.