Nothing but positive experiences so far
I've been to Flex three times now, and have had a great experience each time. The first time I went, was my first ever to a bath, and being a Friday night, I was completely overwhelmed. After about 30 minutes I got into the swing of things (not literally) and made a lot of nice connections. The second and third times were outstanding.
I found the place to be suprisingly clean - my feet didn't get dirty dispite walking around for hours cruising the rooms. The dark areas were active, and the saunas, pool, and whirlpool were clean and well maintained. I found the staff to be curt but friendly enough, even taking the time to explain to a newcomer how things worked.
All in all, I really liked the place. I agree that it could be more crowded but the days I went were very nice weatherwise, so that may have kept people from going. Give it a try.