You know, I can't really hold a bad opinion of James; I do like him as a character...
Well, from the sound of it, yours may be the case of a Lily-Snape kind of friendship, but one that actually worked out and survived!
To me, one thing is mantaining an old relationship in front of new people who don't understand it and one thing is making friends with someone I have apparently nothing in common with; you need some lucky circumstance that makes you realize your presumed nemesis is very alike you, otherwise, there's no chance you'll willingly go to him/her offering friendship...
Both Sirius and Snape never really grew up: one because of Azkaban, the other for his own, messed up personality. We wouldn't like them this much, if they were reasonable, balanced adults.
(And yes, I do love Sirius too!
)
Glad you liked the piece