He's got great scoring instincts and has a good international body of work where he proved quite a bit in the Olympics, and I do think he'd make a fine late 1st rounder. I wouldn't really consider him at #23 (I think there's too many flaws to consider him here), but at #31 he could be a nice pick and a potential steal.
Mills can potentially thrive in the league as a push-the-pace scoring guard, and in particular he's a really aggressive scorer who piles up the three pointers and field goal attempts. His game might actually look better in the up-tempo NBA, where he's playing along better players and shots might come more freely, so that's definitely points in his favor. His stats are actually quite similar to Aaron Brooks as a senior, and he's only a sophomore so he has more time to develop.
But I do have several reservations--while he's jet quick like Brooks to make up for his 6'0" height, he's only at 33% at college three pointers and doesn't get to the line much at all for a point guard, so he wasn't very efficient when he was doing his scoring. Also, he has one of the worst assists/field goal attempt at his position, so his ability to rack up assists is deceiving--he needs a high usage rate to do so. That's why, as currently comprised, he's not quite like Brooks--Brooks was a far far better three point shooter and showed a fairly better assist/field goal attempt ratio coming into the draft, so Mills needs to improve upon this in his transition to the NBA--the three point shot will likely be a big part of his game (it already comprises 50% of his shot attempts in college) and he needs to show that he's at least a decent passer as well.
But again, I think he'll pull through--he didn't have the best teammates at St. Mary's (except for Omar Samhan), he's a smart/heady player, he's proven himself at international venues, he can really really steal the ball, he's still very young, and most importantly, he protected the ball well (had a low turnover rate). I think he'll at least be a decent role player in time, sort of in the Aaron Brooks mold, as he'll improve his three point shot and passing. But with those two in question still, not quite worth the #23. #31 is optimal if we really want him as a push-the-pace PG, but reality I think he'll be drafted somewhere in between (like Brooks was).