I don't know about how to get you into the US store, but there are some free comics once you're into the Playstation Store that you can download on a PSP to check it out. Once they're downloaded you access them under "Extras" in the PSP's main GUI. I grabbed a couple ("Dynamo 5" #1 and "Atomic Robo" #1), just to check it out. The interface isn't bad, but there aren't any real view options. You advance or go back with the right/left controls, and the comic is read by moving from panel to panel, or by jumping to two or three points in a full-page panel (to allow you to read the text and see parts of the action up close). Up and down let you view full pages, forward or back. If you've ever used Marvel's online comics service, it's similar to the "Smart panels" reading mode there.
It doesn't look half-bad, you just lose a bit when you don't get to view a two-page spread all at once in glorious full detail (only in fragments or as a tiny thumbnail on the PSP's screen).
The prices are mostly US $2 per comic, which is a bit of a disappointment when it comes to some of the older comics they offer. I mean, sure, it's neat to read Fantastic Four #1, but for $30 or less you can hit Amazon to get "44 Years of the Fantastic Four" on disc in PDF format, including the fifteen FF comics they're selling on the PSP for $2 each. Not as convenient to read on the PSP, true, but it's still quite a price difference.