Here's my my take, since surely the internet is desperate to hear what I think:
--We simply don't yet know why he walked...seems possible that he had mental issues, in which case he shouldn't and probably won't be prosecuted. I doubt he was intending to join the Taliban or Al Qaeda, because Americans HAVE joined up, and they aren't held against their will. If he was sane but walked for stupid reasons, the military will likely say 5 years in Taliban custody is worse than he'd have gotten from a court martial.
--I think the trade was problematic. It tells the Taliban that hostages are the way to get things they want, and puts 5 bad guys back into circulation, with hero status to boot. It also seems to have been illegal (although since we now seem to have the Imperial Presidency Nixon dreamed of, it probably won't matter.)
Even Stanley McCrystal (no liberal icon, he) is saying that we simply don't yet have the evidence to judge him, and that the principle of not leaving anyone behind is more important than blaming him.