I tried both Crossover and Parallels, here are my thoughts...
Crossover
The setup can be a bit confusing at first, but once you get the concept of bottles down it becomes pretty simple to configure most software to run.
As for actually running windows apps, well it failed miserably. The first program I tried was EVE Online (I followed the specific instructions), after hunting around for an obscure DX file the game wouldn't even let me in, I figured this was okay since it was a bronze rated program after all.
After that I tried Steam, which was rated silver and really should run *almost* perfectly, well, it was even worse then trying to run EVE, it was SLOW as hell, would constantly freeze up and crash, and this was the steam application, I didn't even try to run a game.
Parallels
Tried it, ran slow on my machine (3+ minutes to even get into XP), even slower then Crossover in even trying to move the mouse on the desktop. The initial configuration also takes about ten minutes and decided to add a bunch of stuff to your bootcamp partition.
Needless to say both will be getting the swift boot shortly, both programs add a bunch of crap to your folders (there was literally like 6 EVE folders and 3 Crossover specific folders), and it doesn't come with an uninstaller. Parallels does however.