I have heard that they tend to run hot and noisy, but to be honest I've never really heard much about the drives failing. Thats not to say that they have never failed before, but WD probably does some damn good testing to make sure they can handle 10k RPMs, and the drives have been out for quite some time now, so I don't think failure should be on the top of your list. Depending on the case you have, most of the HDD bays are positioned by the front intake fan, and as long as you have air movement on HDDs you should be fine.
Edit: The raptors may offer a bit more performance, but I highly doubt you would be unsatisfied with a WD640.