I have been using the ComiXology reader for about 10 days now. I've only used it for free comics so far. I read there were issues trying to use it for paid comics, but some have had success sideloading purchased comics.
The reader works really well. I'm amazed how the graphics come to life...
LG G2 - Bricked
LG Nitro HD -- CM11
Two (2) Asus Transformer Primes + Dock -- Hairybean 2.31
Nexus 7 LTE (2013) Stock