As a first time user, probably the two biggest things that will stand out are:
1) you have to buy $20 store credit to start
2) shipping starts at $3, then $0.25 for each card thereafter.
Now, having said that.....with the beginning $20 store credit, you then have the opportunity to submit offers for cards, so there's the chance to get them cheaper than they are listed. You'll see the Beckett high BV, as well as whatever the seller is actually listing the card for, which gives you an idea of how good the pricing is. You can quickly blow through that $20. However, you can't submit offers if you have no store credit to cover the offer price.
On the shipping, it can seem steep if you're buying really cheap cards (under $2), but if you're buying a lot, you'll find that the prices really aren't much different than eBay, and you quickly get to the point where the shipping for larger lots is much cheaper than most eBay sellers. Also, once you've established your account, you can leave the cards you buy at COMC's facility and wait to ship them all at once, saving a lot in extra fees.
If you bought a $5 card today and had it shipped to you, that would be $8.
If next week, you bought another $5 and shipped it, it would be another $8 or $16 total between the two purchases. But if you didn't mind waiting...you could leave the first one in stock in your account, and the wait to ship the two together and only be charged $13.25 for the two $5 cards.
But I would highly suggest, before making the plunge, do a lot of searching on the site. See what all you might be interested in buying. Count up how much it would cost to ship to you. Then try to find those same cards on eBay or Beckett or Sportlots and see how much the shipping would be then. And with COMC, you ALWAYS get your cards in a top loader and bubble mailer or better. No PWEs for $3 shipping like on eBay.
Above all...I highly recommend taking the time to read this whole thread. Every question you have is answered here somewhere.