My friend and I joined Bangkok Night Bike Tour and we enjoyed it a lot. I have been to Bangkok a few times and basically quite familiar with those tourist must-sees but it's all new to my friend so we joined this one and I got to see another side of Bangkok. It's pretty interesting to see the usual hot and crowded Wat Pho and Wat Arun at night, all quiet and without the glaring sun!
This tour is not physically demanding at all but we did go through some traffics and crowds (cycling through the crowds in Royal Garden was kinda fun). People on the tour occasionally fell behind and the guide had to cycle back to find them. I guess it would be ideal to have more than one tour guide when the group is relatively big.
A word of advice - don't panic in the traffic. Thai drivers aren't gonna run you over. I had cycled in Asia and sometimes you just have to ignore them and cross the road or you are never crossing that road. Just stay calm and alert but there's really no need to panic and go so slow that you can't balance on your bike.
Oh and when you are in a tour with other cyclists following you, don't stop abruptly. Give signals - hold your hand up and clench a fist when you slow down. Of course you should signal your turns and stuff too.
P.S. Coconut ice cream in the flower market was awesome.
P.P.S The tour guide speaks good English - unlike the one I had with the Spice Roads cycling tour.