Out of the "1200" cards I've submitted, the service charges have been roughly $200. I like catching show promotions and have utilize a couple of those. The initial start-up is typically free, or extremely discounted. If I compare that price to fees on eBay, it's fairly comparable. But, I'm not responsible for individual shipping fees and the overall hassle of packing, taping, and getting to the post office.
I've sold less than 300 cards, but the average price for a sale is just over $3. The majority of my inventory is not super high-end and I've found that parallel sets are my biggest sellers: people filling in the holes of a 400-card parallel set. They also offer a system to sell your cards on Amazon and I've had a few sell through there as well.
Cashing out is relatively pain-free and I don't think I've waited more than 72 hours ever. There are service fees, of course, but when I'm selling stuff, it all works out. Payment is made to your Paypal account, or you can keep a running credit on their site.
In talking to other sellers at shows, a lot of people are into flipping cards on the site. They'll scour through the listings and purchase cards at the advertised price, then re-sell at a newer, higher price. I've done it a couple times, myself, but nothing super high-end. Maybe $5-10 at a time. I did see someone buy a Bernier Young Guns RC from me for $15 and then double the asking price. It's kinda cool you can track what happens with your cards afterward.
The cards I've submitted are typically inserts, autos, jersey/patch, or rookie cards. Base set cards, from me, are fairly rare. I find the prices for base cards on the site to be too high and would rather sell items with more value.