Offensively, he's exactly what his numbers suggest: he's willing to take a pitch or work a count, and he has some power. The batting average is always going to be so low that it drags down a lot of his production, but for a catcher, he's not a bad option.
People are generally pretty down about his defense, specifically his ability to throw out baserunners. He's below average in that regard, but his numbers the last few years have been especially bad. Maybe that's something that will move closer to his career average, maybe he's declining. He threw out 33% of basestealers his first three seasons, then went under 20% the next three. My sense is that he's not great at it, but it's also not a situation where it was so bad that opposing teams gameplanned around it.
He slugged .484 as a backup last season, so its both a small sample, but also an impressive number. He's going to look very bad at times because thats what low batting average guys do. You'll be watching a game in July and a graphic will pop up that he's 1 for his last 32 or something. But I also think that he could be ready for a .250/.310/.480 career year.
As a personal thing, I think there is more to a catchers defense than throwing out baserunners, so if he handles pitchers well, I'll live with a poorer arm. If he pulled off that 'career year' I'd be thrilled. It compares very favorably to Rod Barajas .226/.258/.403. In fact he has never had a OPS+ as poor as Rod's 73 from last year. They seem fairly similar offensive players, Buck gets on a bit more, Rod might hit a couple more homers. Buck is younger, I would guess he'd have the better year next year. I'd also guess that Rod will get a bigger contract.