I used to pay $15 a month just to have that feature. Now that it's free for everyone on COMC, I think everyone should use it in some capacity.
It isn't just auto-accept. You also get free auto reject and you can set a minimum offer. While you're there, you can also turn on the ability to cross-post to Amazon. On COMC.com (the new site) you can also set your account to "Advanced Buyer Mode" (or whatever they call it), so you see the base prices (as on checkoutmycards.com) without the 25 cent handling fee added.
This gives you a variety of options to support a wide number of selling strategies. You can price your cards super cheap and reject all offers. You can offer 50% offers, but only if the offer is at least $x. You can allow offers as low as 10 cents. You can automatically accept offers that are close to your asking price and consider other offers, etc.
As a buyer, one thing I value is certainty. If I'm going to slog through 1,000 pages of listings to make a handful of offers, It's nice to know that the offers are going to be accepted before I make them. If your price is as low as you're willing to go, I'd like to know that in advance as well. And if I make an offer, I like to have it accepted right then and there, rather than having to wait up to 2 days for a response.
The main drawback of the auto-accept / auto-reject / minimum offer is that the same setting applies to every single card in your account. So you can't say "50% off all cards except those which are already marked at 80% off of guide" or "no further discount on cards, except for Hockey" or whatever.
I would personally prefer it if we could divide our port into several smaller ports, and apply different auto-accept rules to the various sub-ports. But I do like the fact that everyone can now use auto-accept for free, and I would encourage anyone who will accept x% off their asking price to come to the "Spam Your COMC Listings" board and tell us what deal they will accept.