Devine was drafted last year because Schurholtz saw that the Braves' pen was in serious need of some help. He was an excellent closer in college and people were talking about him for the closer job in ATL since the day he was drafted. He's had an excellent spring, and if he doesn't break camp with the job (I'm pretty sure that it'll be Reitsma's job to lose, but I don't think Cox will cut him too much slack), I'm confident that he'll be closing for the Braves at some point this year. My best guess is that he'll be the closer by mid-year, but there are no guarantees.
Another guy to keep your eye on is Oscar Villareal. I think both Devine and Villareal are more likely to be closing for the Braves at the end of the year than Reitsma.
Another guy I've heard people mention is Anthony Lerew, but I would be disappointed if they moved him from a role as a starter.
Anyhow, if you want a Braves closer, I think I would take Devine, Villareal, Lerew in that order. I just don't have enough confidence to recommend taking Reitsma, since I doubt you'll get more than a month or two of saves out of him.
edit: oh, and I'm 95% sure that he'll be on the roster come opening day. He's one of the best arms they've got, and it's not like he's got anything left to prove after pitching his college to the CWS.