It depends. I rely on them a lot for shoes, because I have fussy feet and also like to do the bulk of my shoe shopping online. You do have to read them, of course, since as has been mentioned above, the ones that talk about how they don't like the color or style or whatever are not very useful. However, I do find them really useful when figuring out things like whether the toe box will be wide enough, the heel narrow enough, calf width, that kind of thing. It does also help, of course, that Zappos tends to have like 8 million reviews for many of their shoes, so I think you get a pretty good idea of how good/comfy something is. And honestly, there are so many shoe options available that I sometimes need to winnow them down by some criteria anyway.
For clothing, fit and stuff is a lot more subjective and I tend to find the reviews less helpful, although I do still read them and they can be helpful. Lots of consistently bad reviews might make me rethink purchasing something, but one or two poor reviews probably won't change my mind unless they are specifically referring to something that I know would be an issue for me.
To be honest, I find the reviews I get here at YLF to be the most helpful! Probably because I can ask specific questions like "how wide is the calf on a size 8?" and if someone has it, she will probably go and measure hers for me, since everyone here is so nice and helpful. =) Also seeing things styled in such fab ways here does definitely influence me to buy things too!
