Don't do it for comics, man. Just don't. Not unless you are willing to give Comixology your credit card number direct. If you want monies to be taken from your Amazon account like I do, Comixology won't allow you to buy everything that is available. But if you are willing to put your number out there, then you can get pretty much anything. The format for reading the comics onthe Fire is wonderful and pretty easy to use, I just felt ripped off that I could read the first three issues of any new 52 title, but that was it. Not unless I gave Comixology my credit card and I wasn't so willing to do that. Also keep in mind that the Fire memory is limited. Unlike the iPad, you won't be able to load the device up with comics and leave them there. You'll have to keep taking stuff off. Comixolgy does have a cloud though, so you can redownload stuff over and over again.
If you have the money, I do think that for comics you'd be better off with an iPad only because you'd be able to leave more of them on the device at a time. 
I do have friends who love the device but they mostly use it for books and for that I already had a Kindle DX which I prefer to read on because it isn't back lit like the Fire. Plus I can load the DX up with books and leave them on there. 
