I'm in the review time as well, although I'm giving them out :
From what I've seen at our company, the self-evaluations are used for 2 things.
1. If there is a serious problem with a manager, supposedly the self-evaluations can be compared to the real evaluation, and then someone else can see which seems more accurate. Never heard of this happening.
2. It helps remind a manager of what happened 9 months ago.. really just putting the burden on the employee to remind the manager of what cool things they did earlier in the year.
So in our case, it really is nothing but a chance for someone to remind me of what they did. I have actually changed a part of the review in the past when I was reminded of something big that happened the first few months of the previous year. The last few months hadn't been very impressive, but they'd kicked some serious buttocks earlier in the year, so I bumped them up a bit.
Now what's funny is that everyone up the tree above me demands that I do these on time, yet I haven't had a review in years.
Oh, and I view them as a bit of a waste of time with both "really good" employees, and "really bad" employees. For the good ones, I have a tough time saying anything other than "Please please don't quit, you're the reason our team doesn't perish in flames". For the bad ones, I have a tough time being positive, because saying something like "You'll never be a good programmer no matter how hard you try, though if you work really hard we might keep you around." just doesn't inspire people.