I have used WHMCS as i was thinking of setting some reselling up, but i backed down as i didnt think my knowledge was big enough to run a wee bit of webhosting although it is something i'd like to do.
However been a user and reading up on it, i found it rather straight forward to use and it was packed full of features. I would give thumbs up for this bit of kit. You can find a comparison chart here:
http://www.whmcs.com/comparison.php
My recommendation would be to get the demo downloaded and see what you think.