I love reading on the Comixology app! I used to do it on my iPod Touch (in Guided View), but two years ago got my iPad Air and that's perfect (slightly smaller than the issues themselves, but still larger than say a Dark Horse omnibus format, so you're not straining to read a full page, or even a double-page spread in landscape view).
I've bought tons digitally, almost entirely on sale in some fashion. When things are 99 cents each (or $3.99 for a collection), then I'm much more willing to take a chance on it. And since I've so much to read, I can be patient and wait for sales. You know that some sales will arise when movies come out, when big crossovers happen, during SDCC and NYCC, and during the holidays. And Marvel has at least two sales per week.
My only complaints are about reading order. I love reading crossover events in chronological order, so I'm jumping around between series a lot. Plus, when you're offline, it often doesn't tell you what the next issue is in your story. Or, if you're reading some of a series in Digital Collections and some in individuals issues (case in point, I read Jem and the Holograms Volume 1, then Volume 2, and then issue 11+ individually, before Volume 3 was released), then it tells you the next issue is Issue 1 (each TPB available, then each issue available).
So I would love the ability to customize a reading list. I have written them about it before, and may again.