They're good and bad.  On the one hand, you can set a reserve
so that you don't lose your initial investment on the item.
I have seen cel auctions specifically where the reserve was
the amount the seller would take to end the auction early for
a flat deal.
On the other hand, reserve auctions do scare off a lot
of folks who may think the reserve is very high.  That and
I've seen lots of auctions where the ending bid didn't make
the reserve price.  In which case the seller often will say
"ah, what the heck, I'll let it go for the final bid price."