http://www.blackberryforums.com/ring...-4-2-only.html
It simply unlocks the other carrier themes. I honestly have these extra carrier/vendor themes on my Pearl now because I test the ALX files I modify before I post them for the rest of the world. But some people like the other carrier themes (I used to love the Vodafone Today theme, for instance). Personally speaking, if you like what you currently use, they aren't needed and take up space. There is a white-based Zen theme (exactly like the default blue-based theme) with pink-accented lettering that you may like, though.
As for the upgrade, I've only had it on my device for a few minutes. I'm not really all that picky with some of the 'issues' that others have had (for example, the myFaves issue doesn't bother me one bit). Aside from me answering a call and the other person not being able to hear me until after I say 'hello' four times (or they decide to talk), then I've not really had any issues with the 8100 yet (although I will say that my thread on the Lock/Unlock issue does outline one of my bigger gripes about OS 4.2).
Keep in mind that I've only used the OS you have loaded for approximately 5 seconds, though. I've been using the two previous versions since before they were released by any carriers, and now I just upgraded to this one (no source for this one, unfortunately).