Meh. CGC is pretty much the ONLY comic grading company. They're respected amongst the comic community, but like anything else, it comes with a catch: you can't open the slab without voiding the warranty.
You can't read the book, which to me, defeats the purpose of owning a comic book, and if you remove it from the slab and discover that the 9.8 NM+ comic actually is missing a page, you're screwed, because you've opened the slab and voided their warranty.
Also, like with baseball cards, buying something in a CGC slab means you're going to pay a premium.