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As a satisfied owner of a jailbroken iPhone, on which I have read many e-Books (but no comics), I strongly recommend you don't.
You need both a filesystem you can access and intelligent software that is allowed to access it. The iPad has neither.
Expect tons of non-obvious gotchas and limitations.

Do not buy unless you can get a chance to borrow an unit and verify for yourself that you can read 500 pages of diverse material without swearing a lot about all the things the machine won't allow you to do because it was optimized to display albums with 30 photos.

Slightly related, I use a widescreen display in a vertical position to read scans myself, using Xee (a mac program done by the same guy who did the text board and image board software saovq runs!)
It's not mobile but really convenient.
You could reproduce this sort of setup with a half-as-expensive-as-an-iPad laptop rested on its side, or one with hinges that can open at a 180º angle. Almost every laptop produced in the last 3 years allows you to change the display rotation for the whole system, often right from a system tray icon, so would not be limited to software that supports a default rotation.