Comiczeal ($1.99 at the Appstore) has evolved into something that is a joy to use. It's fast, stable and stylish, complete with bookmarks, longboxes, and automatically "bag and boarding" comics you've finished reading. A free desktop component converts CBR files into a slimmer format that'll look good on the iPhone screen without sucking up all available memory and bringing the iPhone to it's knees.
Of course, there are a lot of comicbooks on sale at the App store for 99 cents from
iVerse and
Uclick. Lots of good stuff, including Star Trek, Bone, and the Teenage Mutant Ninja Turtles.
I've got tons of Marvel comics in PDF format, from the DVD-ROM collections that
GIT Corp used to sell a few years back. Sadly, I can't read them on the iPhone because the files are so big that the iPhone PDF reader always ends up crashing before I get very far. I haven't yet found a program that can inexpensively slim the files down for the iPhone, so I'm considering getting a tablet PC instead (the $500 CTL 2go Convertible).
If anyone has any recommendations for programs to shrink gigantic graphics PDFs to a more iPhone friendly size, I'm all ears. PDF Annotater isn't an option since I don't have a Mac (and unless they've built their own PDF rendering engine from scratch, it would choke on huge PDF files the same as any other iPhone app).