I've got the V-4. It's my first one piece, so I can't say whether it's more or less comfortable than any other - but I don't notice the suit while riding, so that's something.
It's liner is not removeable, and there's a small perforated area in the lower back. Other than the airflow through the keprotec panels, that's all the venting you get. Probably not the best for hot weather riding. I don't know if they offer a perforated version or not...
Overall, I like the suit a lot. It seems to be of high quality, and I don't have any complaints.