As far as digital comic readers go, it's pretty impressive. The Store has launched with 400 comics, with each comic costing anywhere between $1.75 and $2.45. There are also a heap of free comics available to download to get you started.
Comics are purchased through the Playstation Store on the PSP itself, or via the MediaGo PC Suite and sideloaded onto your device.
The comics themselves come from Marvel, IDW, Rebellion and Disney, with a combination of both old and new issues available.
I don't know about you, but I'm actually really excited by this. I've been a bit disappointed with the PSP's games offering recently - especially compared to the raft of great, cheap games available for the iPhone - but being able to read comics this way is a nice new feature I think I'll really enjoy.