@icbrkr:
If they work described they will work with your Amigas just fine, although you should not expect much better picture quality than what the average video-grabber-card for your PC would achieve.
There was recently a hack released on Aminet describing how to add an S-Video port to your Amiga. That would without doubt increase the picture-quality you get from a box like that tremendously compared to when using the composite signal. Unfortunately
www.aminet.net is down so I can't give you a link.
(edit):
Found it unpacked in a public_html dir of mine, check it out
here.
One thing to keep in mind both if you are using a box like this or a real flickerfixer/scandoubler is that it will most likely not produce a resolution that is the native resolution of your TFT. This will make the TFT scale the picture, which probably will be ok for games, but for productivity work it would be advisable to run it on a CRT.
/Patrik