I like Sirius, too, and I think that he is quite undervalued by many readers (and I don't mean those who just plain hate him). From all that we know from his behaviour in GoF in the first place, but also from the background information we got in PoA, Sirius could have been quite a huge asset for the Order if only they had let him and hadn't locked him up in that horrible house to deal with his horrible memories all by himself. He never really had a chance, the poor man, and still, he was trying so hard. I greatly appreciate the fact that Sirius was the only adult in OotP who took Harry's issues seriously and genuinely tried to help. He was the only one who wanted Harry to have information on Voldemort and supported his idea to practise Defense against the Dark Arts. Whether or not encouraging Harry to form DA was a sign of recklessness is immaterial - Sirius, in spite of not really knowing Harry, realised that Harry would try to get information and carry out his plans in any case. Something that Dumbledore conveniently overlooked.

But, I'm sure you know that all yourself *g*