An interesting signing of a guy who wasn't on any of our radar screens, I don't think. Chavez was the Mariners' Opening Day left fielder last year, didn't really hit well, but fielded well, which is pretty consistent with his career path...he's a guy who is considered well above-average at all three outfield positions, and who can't hit well enough to hold down a regular job, sporting a career 679 OPS and .300 wOBA.
As a well above average defensive outfielder who can run, he'd appear to be a great fit for the 25th man job on the roster, except that he's a lefty. I suspect that he'll get a look in spring training for a bench job, though, and will be the primary option in centerfield during the season if Julio Borbon misses extended time or if he falls on his face.
UPDATE -- Per ESPNDallas.com (there's no byline on this), Chavez is not expected to be ready for the start of the season, as he's recovering from ACL surgery.