I really think it is difficult to not root a phone once you have done it, barring a traumatic experience or brick on the first try.
These are my thoughts:
The first phone I rooted, I rooted because the phone needed it. The phone was slooooowwww to update, was missing features, rooting provided some bug fixes and got rid of that pesky bloat. This phone is pretty much prefect out of the box. I really can't see anything wrong with it to root it for the reasons I did my first phone.
With that being said; rooting this phone really shows you how awesome of a phone Samsung has made. This is the best phone available right now. Sure it isn't pure google but who cares by rooting you can remove all that and run pure AOSP if you want. I love being able to back everything up with root, I love being able to modify or uninstall system apps or features, and I love have choices with my phone.
I would almost gauruntee that if you love your Evo rooted, you will love your Samsung GSII rooted.