Personally, I took step I USMLE, but not step II. I met many people on the interview trail who only took COMLEX and got great interviews and ended up matching to great programs.
If I was to do it again I probably would have taken both USMLE steps - a few of the more academic/competitive ob/gyn programs were confused by the fact that I didn't take both... and other than cheapness and feeling I did good enough on step I to make the "cutoffs", I didn't have a good reason... 🙂
Overall, ob/gyn is very "COMLEX receptive", but especially if you want to be competitive to academic programs, I'd recommend taking USMLE.
Hope that helps 🙂