Good for practice

The CATs provide a good estimate of the actual score.
The verbal section gives you a higher score as compared to actual GMAT. I also feel the verbal on actual GMAT is different from MGMATs. The quants section is harder. In my opinion, the reason is not that the questions are harder but I was good with Quants so it wasn't an issue for me. On actual GMAT, you'll score 2-3 points more than on MGMATs.

The final score I got in my GMAT 1 was the exact average of 5 MGMATs I took.

Note: You can also reset the 6 MGMATs after completing all of them but the score you get on the retake won't be an accurate indication of your preparation.