If you want to take the manga with you, n and hav a DS or PSP. Voila.
DS will require of those special cartiages though. I saw one where you just stick a mem card in it, and plug into ur DS card slot (its the same size card). Think it was called R4DS.
Anyway, thats cheaper in the long run than actually 'buying' the manga.
Why buy when you can download as Hales said, works for both manga and anime ^^;
(and everything else)
I have one of those, it's not that great for reading manga though. Then again, DS isn't that great anyway.
Haw haw haw.