are they any good to help firm up stomach

I had one and to be honest it didn't do anything and it was a bit of a faff. I found that doing sit ups with one of those ab cruncher/sit up cradles gave better results.

They're alright if you're already quite trim as they can work on the muscles through the little fat.

If you have some blubber down there on your stomach, there is no point using them. If you want to lose your blubber - go jogging and burn off the fat.