So far this year, the Giants have a Defensive Efficiency Ratio (that's the percentage of batted balls turned into outs) of .732, best in the National League and second to the White Sox overall. By the plus/minus system, the Giants' fielders have made 22 outs above average, best in baseball.
Can they keep it up? Probably not. For one thing, the best-fielding team last year, San Diego, finished with 91 outs above average and a DER of .703; the Giants are way ahead of that pace. For another, they've been playing three regulars (Feliz, Winn, and Aurilia at first) who have good gloves but don't really hit enough for their positions, and Bochy is already shaking that up in search of more runs.
But still, this was a good defense last year and it looks to be a lot better this year. Not bad for a bunch of old guys.