It definitely can cut both ways. In some respects they may seem easier on priors early on in the training cycle because they're spending most of their time correcting non-prior candidates on things that the priors already know. However, they're going to expect more out of you in many respects, such as the early billets that were mentioned earlier. Also, taking the initiative to help out other candidates will help you out a lot. On the flip side of that, if you have a lot of experience and you aren't using that to help out other candidates, that can raise serious questions about your initiative, motivation, desire to be a team player, etc. Also, even if the staff might not be as consistently harsh toward you as the other candidates, there's a good chance that they'll have a smaller tolerance for major mistakes coming from priors, particularly if they see you as not working very hard or taking OCS very seriously (there's a pretty good illustration of this in part two of that marinecorpstimes.com "class 186" feature). Overall, though, being a prior is an asset as long as you use it properly.
Bear in mind this is all coming from a non-prior, so take it for what it's worth.