i hear cosmos is one of the best, you can choose to have all your stats and stuff put on a webpage (free) and it keep track of hit rates ect, i believe you can find it at thotbot.com you can check the WoW forums for more info there.
Some people find them to be too much of a hassle to use or just dislike using one altogether. I personally love the convenience of it have have yet to run into any problems. The only "hassle" it has caused me is that I have to reinstall it every patch. It's really your choice if you want to use it or not.
I tried cosmos out and it had some nice abilities like being able to record your average DPS and average miss rate and things of that nature, it also allows you to have like 600 tool bars on the screen at once, and has built in mini games you can play with people in game.
I found a few bugs during the beta that caused some problems for me and uninstalled it. Since then i've become accustomed to the Shift 1 Shift 2 Shift 3 for my tool bars and have no need of another bar.
If your not looking for all the bells and whistles that Cosmos comes with I may suggest trying one of the other less flasy UI, you can find a list of them here.
i'm looking into getting one...every time i send my pet in (ctrl+1) and then try to back off and attack by pressing S...i end up turning my effect sound off like 80% of the time...sound off is ctrl+s. annoying
I tried it last night...and so far its working great. I had only one crash, which is frequent when i play for few hours and my memory is low (I think wow does poor memory management). When I restarted my comp and loaded wow, most of my cosmos settings were reset to default. I guess it was due to crash though.