Over the last month he's hit .296, with 17 R, 1 HR, 12 RBI and 6 SB. That's not superstar level, but that's good production for a rookie owned in only 9% of Yahoo leagues.
Fangraphs likes the guy, and thinks he can be productive this year (though aren't big on him as a keeper, on which I agree):
Fangraphs wrote:Tabata’s numbers have been pretty close to our expectations, and he should only get better. His BABIP may be over the .300 mark, but should actually be 35 points higher thanks to his speed and tendency to keep the ball out of the air. He’s already flashed good speed on the basepaths, stealing 9 bases (13 attempts) in 158 plate appearance. He doesn’t strike out much, and has shown an above-average walk rate. I like him a lot as a steals guy, but I’m not a fan of keeping guys just because they can steal bases.
(I'd post a link to the Fangraphs blurb, but something about the url is confusing the BBCode in phpBB)