Don't bother with the Marvel app unless you just want to filter out the clutter of non-Marvel stuff, it's basically Comixology but only with Marvel titles. Also watch out if you're using it on a Kindle Fire, as apparently you can't buy any DC titles through that version of the app for some reason.
I like it as an app, big fan of how it converts comics to fit on my phone's screen and I really like the way it flows.
What I do hate them for is constantly putting great titles on sale. Like right now they've got a big Walking Dead sale on with the first 99 issues for $0.99 each (or £0.69 over here), last week they had a load of cheap recent Amazing Spider-Man and Ultimate Spider-Man titles on sale to tie in with the new film. It's getting to be constant "take my money!", which combined with the limited storage space on my phone is going to get annoying fast.
http://www.comixology.com/comics-sale keep an eye out there for new offers, generally new stuff every day or two