I think it's one of the best. It gets a lot of bad reviews from when Amazon bought the company and stopped using in all purchases (so you have you use a website to purchase new comics) and customer service not being great.
I know this is anecdotal but I've had to contact CS twice for them and both times I got responses within an hour, one of them was on a Sunday afternoon, so if it was a problem it seems that things have changed.
I love comixology, and other than MU its my only source for buying comics. Not buying in app might be a pain, but it takes no time at all to hop in safari, make your buys on Wednesday, then pop back to the app.