Previews are tough because you know they're being hand picked/edited. I try not to take too much out of them if I can help it.
I wouldn't say I've wasted money but I've definitely purchased clips that I thought were, well, disappointing after getting them. Can't blame people for it, really. It varies as to why, some have boring camera angles or have unflattering camera angles, or just dull camera work that really... dulls... the scene, I guess. Others have had tickling so light and without bondage that there was hardly any content to them. I'm all for light tickling without bondage, sure, but when the lee just pulls away every few seconds for a few seconds and the ler/lee are clearly not the most comfortable with each other -- again I dont judge, you cant blame anyone for feeling that way -- that he has to wait for her to put her arms back up or her feet back in his lap? Iunno, just, eh. Honestly, and I really mean it, things like that seem pretty rare to me. Even the models that I end up not liking as much after watching their clips. Usually, for a few bucks, Im satisfied enough.
*I want to be more forgiving about the camerawork because I really do understand how rough that has to be if you're by yourself. I suppose I'm just spoiled from the old RT days that had interesting and pretty good camerawork (mostly). He also had the luxury of not always doing the lerring, but those clips usually weren't as great. Man, I miss TiB.