Marvel has a few free issues in their app. I read New Avengers 1 and found it to be a great experience. It’s a little small but readable. Higher-rez scans would be nice to make the lettering smoother. You can double-click a panel and it’ll zoom into that one panel (which must be a lot of work for Comixology to put together for every issue). I prefer to turn off the “letterboxing” feature so that you can see other nearby panels as you zoom into one. It feels more like reading the real issue that way.