Hi there. I have been working abs for years, and I too was intrigued by all the new ab devices. I myself bought the AbSlide, which is a little less expensive than the dolly but the same theory... a device that has you extend out and pull back from that push up position.
I love my abslide, but I have to tell ya that despite the advertisements, these devices are not for beginners. If you don't have a strong back and fairly strong abs to begin with, you tend to put a lot of pressure on the wrists and upper body and "cheat" the move so all it does it hurt you. I do love my ab slide, ($39), and I think it works great, but again not for beginners.
You may note the new trend in exercise technology is the "core stabilization" like Pilates? You can get almost the same exercise by doing "plank poses" (Cathe does these in her Circuit Max video) and you get almost the same result.
I guess what I am saying is I think it is a great exercise tool to add to the tool box if you are already pretty strong, but not good for beginners.