I just got some Neuro-Grips today so I thought I'd share my initial experiences. Firstly, I should say it's unlikely that they'll totally revolutionise the way you train, but they are a 'neat gadget' (if something so low-tech can even be termed a 'gadget') and if you're really into increasing the variety and challenge in your push-up routine they're certainly something you should consider.
Used carefully, they're really not dangerous, provided you've got solid form on standard push-ups and are strong enough not to find them too much of a challenge. A cautious user can practise push-ups from the knees with these or even just work on holding a plank until they're strong enough for full push-ups and you can always place a cushion on the floor beneath your face if you're afraid of hitting the deck. You'll definitely want to finish your set before you start to wobble too much.
Overall, I think they're a pretty nice bit of kit, however they are rather expensive for what they are (especially if you have to pay import tax on top of international postage!) and there's really not that much difference in feel between using these and doing push-ups on low-hanging gymnastic rings, although they take less time to set-up and you'll avoid the chafing that can be caused by the suspension straps. I can see myself getting a lot of use of of these, but they're definitely not for everyone.