He strikes out a ton, leading the National League this season. That can be frustrating. And has been around these parts. But he's actually pretty good.
His offense appears to be just about league average (career OPS+ of 98). That plays really well in the middle infield. He's got enough pop, with a career 162-game average of 21 HR. He can steal bases, with 34 on 45 tries since 2011 and a career 162-game average of 18 SB.
And then there's his glove, which actually appears to play better at the generally more difficult shortstop position (although it is his natural position) than at second base. The sabermetric stats back this up despite what is so far a tiny sample. His UZR at SS is already above 6 runs in 256 innings at SS. (Or, 33 runs per 150 games.) Fortunately, the eye test supports the small-sample sabermetrics. Espinosa is an above average middle infielder. Even at second base, that's a plus.
What do you get when you cross an average hitter with a good fielder in the middle of the diamond? A very good player. Espinosa last year was worth 3.5 wins above a Triple-A-type replacement player according to Fangraphs (just behind Rickie Weeks), and is 2.9 wins above replacement so far in 2012 (just behind Ian Kinsler). The same numbers at Baseball Reference are 2.4 last year and 1.9 this year, but the standards at the latter site are higher, and a number of 2 or more over a full season indicates a player who is a starting-caliber Major League player.
Espinosa is on schedule to enter arbitration in 2014, and free agency in 2017. An extension at some point before he becomes a free agent would not surprise or disappoint me.