I've had pretty good success. Have moved over 1k cards that I had an impossible time trading or selling on sites like HI or eBay.
The beauty of COMC is that they do all the work, but yes.... you're paying listing fees up front, so it can be costly.
Myself, I've been going through a lot of older boxes of stuff (90s through this year's stuff) and I've been classifying things in one of 5 ways:
1. It's something to keep
2. It's something good enough to try and use a trade bait, or an eBay sale
3. It's absolute junk, and I'll file it into a black bag for the man to pickup on Monday
4. It's unmovable base, but modern, so I can give it away to charity (thanks breakawaysc!)
5. It's too good to toss, hard to move, time to send to COMC!
The 5th group, it's consists of anything from the following:
Rookies, Parallels, Star player base cards, Autos, GU, Inserts
I've done pretty well at moving a lot of that stuff. Set your expectations at a realistic level though. Remember you're paying 25 cents to list, and COMC charges 25 cents (on top of your asking price) the first time a card sells (flippers don't need to worry about the extra 25 cents after it's sold the first time).
So, I've sent loads of Score Hot Rookies or Victory Rookies (common players) in. Do they sell?? YES! Do they sell fast? No. Do I make a lot? No. I just sold a Johan Motin Victory RC. It sold for 60 cents, 25 of which went to COMC, 25 of which covered the cost of listing it.... and I made a big fat dime! That's 10 cents more than I had before, it's a card that really isn't worth anything to me though, so happy to have done it.
I find if you're selling autos (not BAP or Scoreboard type stuff though) if you want less than $1.50 for them, they move really quickly. If you price GU cards for $1, they fly out quickly too. The margins are not huge, but it's easy money on low end stuff, and I just build up a bank roll and then spend on their site again.
Also, if you've got lots of stuff you're buying from them.... wait until you've got 100 cards to have them shipped. If you ship 100 cards in a month, it still costs only the $3, and you get a $5 store credit.