My husband and I have a Slendertone kit. I think they do tighten the stomach muscles, but you need to do it every day, and don;t expect miracles. I think trying to find a second hand one or one from a friend is the best idea first, as the novelty does wear off.
If they do, you really have to use them every day. I have seen absoluely no difference since using mine and perhaps more imortantly, have never felt any difference (no muscle tightness or anything) suggesting to me that they do not work. If you are going to get one, make sure it is fully portable. Mine is quite bulky so you have to sit down while using it, with the result I got bored quite quickly. At higher intensities they are also not exactly pleasant to use!
They *may* strengthen the muscles, but they do nothing to the nerves that controls those muscles, so if you are doing it to increase strength, it's pointless. They are great for losening a tight muscle, though.