45 minutes of spin twice a week for a year shifted 10kgs for me. Without diet change (e.g. still eating moderate amounts of chocolate cake). That took me back to my pre-pregnancy weight which is to say that I had/still have a couple of kilos to shift.
Don't know if the fact that these sessions were at 6.45 in the morning added to their effectiveness but it did feel as if the sheer bloody-mindedness required to get out of bed in (what felt like) the middle of the night burned a few extra calories. I started in November which is not the best time to start something that requires getting up that early. It was hell at the beginning. It took six months to break the back of it (i.e. not feel like I was going to die on the bike). I think after six months my cardiovascular system just got a lot more efficient, it was really great and I still feel the benefits of it. At about the same time the weight started coming of really fast. By the summer all that spinning was burning 1kg+ a month, easily.
Had a cesarian and did manage to get back in the gym three months after, doing a bit of body pump. But it wasn't enough, had to get the deep cardio you get from spin to see the results. Will say again it was hell but also well worth it - I can sprint for a bus, climb ten flights of stairs without fatigue, and there is no way that baby can outrun me now. Would definitely recommend this regime for throwing of the worst of the baby weight in an effective, not too quick and not too slow, way. But it was hell!
If I were to do this again, which is to say, starting first thing in the morning, I would start in March as the getting out of bed in the winter months is really difficult. Should probably have changed my diet to speed things up a bit. It is not bad but not as good as it should be either, far too much sugar and fat, and is something I still need to fix for the long term. Sigh. And a note of caution: there is no point going to spin and coasting along in low gears or avoiding the sprints and jumps - other people did that and while I lost a lot of weight I have to say that the coasters still look much the same as a year ago...
I still do a bit of spin and picked up pump again but primarily get my exercise from playing handball. It is much more interesting, requires strength and agility, and is really good fun. You don't really notice you are exercising which makes a nice change.
Good luck to everyone - don't give up the good fight.