I've been dabbling with it this year more than I have in the past and I'm now working on trading out of my account, so I may have a dissenting opinion...
My overall opinion is that it's another hobby niche. It has it's plusses and minuses and, for me, it's an overall miss. If you want to scratch the pack-opening itch, sure. If you have time to kill trying a challenge or playing virtual pack wars, why not. I feel it's an overall miss is because it add another layer of complexity to the hobby. [I do NOT think it's going to go away.]
My top issues:
(1) Your cards are already at COMC. Say I open a box on epack, I can trade, etc. in their interface, but those cards are already at COMC. Now to 'get' my cards, I have to 'transfer' them from the epack interface to the COMC interface. Then I have to pay COMC to transfer/ship them to me... I've worked with supply chain in my professional life - increasing the complexity of a supply chain through the use of software is a VERY backwards concept. To expect the customer to pay for it is not usually a long-term win. [If I had to make a guess, COMC will eventually take over management of the epack interface. If I were going to go a step further, I'd suggest they offer a 'shipping credit' with the purchase of an epack product. If I go for the trifecta, COMC accounts will eventually allow for trading between accounts.]
(2) The 'community' and trading is frankly a nightmare. I haven't gotten to 100 trades yet, so I'm still 'new' and I get a lot of garbage trade offers. And I get a lot of messages telling me I'm 'crazy' to expect a trade at COMC pricing, or ebay pricing, or any pricing that wouldn't be at least 2-to-1 in their favor... Again, in my opinion, it's the wrong direction to take the community. I've been collecting for 4 decades and, if I'm make what seems like a low-ball offer, I can tell you why based on market factors rather than 'you're new here and just don't understand.' [If the platform shifts to COMC, this is eliminated and the education level of most users is automatically elevated.]
(3) The economics do not seem to be very good. I love OPC Platinum and have looked at that the most - eBox price is $95.99, you will get 0 base (all digital) and ~16 physical cards. To get those physical cards, you have to transfer to your COMC and then ship to yourself (cheapest is ~$8.99 in US - $4.99 + $0.25/card with delivery in 3 months). That's $104.98 for 'the hits' of a box - that's almost $20 more than a physical box and you're not getting the 65-ish base cards and waiting 3 months to receive... [Quick calculation FYI - the average value of an insert/parallel in 19-20 OPC Platinum isn't $8]. I'm not sure how so many people can afford to get kicked in the junk so many times and for so long, but, hey, that's their choice - not mine.
It seems to be working for UD and COMC has hinted at opening an office closer to UD, so their relationship may be expanding. And, as a 'buyer', it's great because it keeps prices down on COMC. I mean, you can get a complete 100-card set of 19-20 OPC Platinum Retro on COMC for ~$8.00. Even with the (somewhat ridiculous) $30 shipping cost, you will still beat ebay prices by a clean $20-$30 for that set and you are getting 'uncirculated' cards.
I don't think lower prices on COMC alone are going to grow the hobby, though... People talk about the 'growth' of the hobby this year on the back of record HIGH prices on cards. It's not growing because I can get OPC Platinum Rainbow commons for 2 cents...