I personally don't like Kaplan's CATs, don't consider them good. Their verbal especially is convoluted, and the scoring algorithm punishes you too much. You can't gauge your progress from Kaplan's tests; they underestimate your performance by a good 50-100 points. Spend some time digging through the forum and you'll see numerous posts about this.
For good practice tests, nothing beats the official tests, GMATPrep. They're free from
https://www.mba.com. The next closest ones are Princeton Review's tests, in Cracking the GMAT. If you do a class, I would do PR over Kaplan. I did PR's online class and it was awesome.
Oh, and be sure to get the Official Guide, 11th edition, for a ton of practice questions. It's a must. It comes free with PR's course (Kaplan doesn't even tell students it exists, which blew my mind), but if you don't take a course, you can get it cheap enough on Amazon.