So I'm fairly new to MCS and I've been consigning/auctioning my slabs with MCS for only around 4 months now and can say I'll never ever again sell my slabs on eBay–I sincerely wish I'd hopped on board earlier. I highly recommend giving it a go if you've been on the fence thinking about it. There was a bit of a learning curve to understand the layout and function of the different sections of MCS.com but once you figure it out the process has been so easy. Honestly the most challenging part of the experience for me is packing up slabs safely enough and getting shipped out. I've found that using pirateship.com for shipping has saved me a little bit on the shipping cost to MCS. I LOVE the fact that once your slabs are in their system (after a week or so after they've received your package), you can look at recent sales data from MCS and GPA to help you decide how to price your books.
I just started to utilize Auctions to move slabs I don't want to hold on to anymore and I've had good results so far with prices I'm happy with. I've got one auction that ended June 27th that hasn't been paid for yet but I'm hoping that will get resolved once the current Prime auction is finished up.
I do wish MCS and their auctions (at least their Prime auctions) were promoted/hyped a little better. I don't think I've seen a promo through CGC official emails yet, whereas I see Goldin, Heritage, ComicLink and Shortboxed constantly promoted. Even a more visible nod on social media would be a good idea. I hope that with addition of Steve Borock to the team, there will be some crazier auctions in the future that will attract more attention.
On the buying/purchasing side of things, I primarily use MCS to purchase all my preorders/subscriptions. Hands down the best prices (including shipping price, packaging, and bags/boards) I've seen so far. I do wish you could preorder incentives/ratios, but that's not a dealbreaker for me. I won/purchased my first auctions just this past week and thought I picked up some steals.