I'm excited about this but tempered until we figure out what role he is going to have. Is he coming to start? Is he going to be the short end of a 3-headed 1B monster? So far we've seen Velazquez play once every 3-4 games despite hitting well when he's played, and Rios has 20 total ABs and we're about 6 weeks into the season. If Hosmer isn't being DFA'd as part of this move, I can totally see Mervis falling into the same situation, at least early on.
That said, its clearly not a temporary call up as he's being added to the 40 man in order to come up. He will get opportunities to play and presumably he's up for good unless he just doesn't play well.