I just went with Kaplan. From what I can tell, all of the services out there are fairly similar. I can't say I drastically improved my score with it, but it did improve, and as a result of my score, and possibly a very aggressive letter-writing campaign, I got into the school I wanted to go to, and am now preparing to take on a ridiculous amount of student loan debt!
One thing that I found nice about Kaplan -- and I don't know that other services don't offer this -- is that their testing center kept long enough hours so that it was possible for me to come in on evenings and on the weekends to take practice tests. Also, the online portion of their homework was very helpful. I can definitely recommend Kaplan, but on the other hand, I can't say anything bad about the other systems out there. Best of luck.