It will void your warranty yeah, but as long as you follow the root guide to a t, you shouldn't even be concerned with your phone becoming a brick.
The warranty becoming void is really your only downside to rooting. And if it needs to be repaired, it's fairly simple to unroot and it won't be water marked like htc devices.
To me, the advantages of being a root user are obviously having access to root only apps. My favorite here would be titanium backup. Which gives you the option to back up/restore apps/system apps with data.
Rom flashing, huge advantage here

. Some aren't into rom flashing, but if your big on customization and tweaking, you'll fit in rather well here.
But here is one down side, if you've never been rooted, if you intend to flash roms and such, you need to read and research on how to do so. Otherwise if you flash something not intended for the Galaxy S2 4G, there's a chance you could soft brick (recoverable), or worse which could end up in a hard brick.
Hope this helps you out some.
