Huh, I didn't expect James and Sirius to have a dynamic virtually identical to that of Fred and George, but I like it a lot. Elvendork! :D :D I also love that they obviously didn't listen to Moody and continued to keep their wands in their back pockets.
On the other hand:
Magic rules, Muggles drool. WTF-ever. I know it's always been JKR's shtick but it's also always annoyed me. I mean, okay, I realise that we have to make James and Sirius REALLY COOL ZOMG!11 but when you continually make the opposite side look boorish and incompetent in order to make your own characters seem awesome, your characters can't really be all that awesome if you think about it. It doesn't take much to be cooler than a dimwitted cop who's especially funny because he's a fatty (yeah, don't even get me started on that part of the prequel; I'd just capslock a lot).
But anyway, that's really meta-meta; I loved this and I do so wish I could tie JKR to a chair and make her write nothing but HP for the rest of her life. I miss not having a book to look forward to, and I miss the HP world -- JKR will always be the master at writing about it, as far as I'm concerned. There really is nothing like reading a bit of HP for the first time ever. ;_;