The 99 designs process can give some amazing results (caveat, only what I have seen when people were asking for feedback on which design to choose). But understanding how the pricing works is still a maze that keeps me from considering it for my own covers. You don't just pay the person you select, as a sort of understand it, you pay the next two some sort of "also ran" money. And I think you kind of double pay the person you select.
See...maze!
If you decide to pursue that option, just really really really make sure you understand the payment structure before you start.
The upside, however, is that you get some great options without being on an equally good artist's waiting list to get a cover done (not uncommon for them to be booked up to a year out).