I'm not a Rangers fan, but I think it's a little early to be measuring up this trade. Right now McCarthy is struggling, but the Ballpark is not an easy place to acclimate yourself to as a pitcher.
I can see why the Rangers made the trade. Both pitchers were top-level prospects, and McCarthy had at least one season of big league experience, albeit as a reliever and spot starter. Danks had little to no experience. The Rangers needed someone to fill a spot in the top 3 of the rotation. McCarthy should have been in the rotation in Chicago, but there was a logjam of SPs there. McCarthy's young, so it's not like they mortgaged the future.
And as you mentioned, Danks just got his first win. He's got a lot of Ks, but it's not like he's been lights out. He hasn't gotten run support, but the Rangers lineup isn't exactly on fire right now. I don't know much about Masset, so I can't comment there.
I think this trade will probably work out well for both sides. McCarthy's too good to be this bad for too long.