The post-O Brother celebration of bluegrass as a warm, humane outlet for musical expression tends to overlook the fact that bluegrass tends to be dominated by technicians who can play the demandingly formalist genre with skill and speed. Like reggae, bluegrass places requirements on its musicians: Too much experimentation leads to a breakdown in the style's fundamental nature. Nickel Creek, a Southern California trio of acoustic pickers in their early 20s, is capable of ripping through classically structured Appalachian dance music as fast as its audience can move. But the band has also picked up the slightly bent aesthetic of its indie-rock generation. While Nickel Creek's sophomore album, This Side, has all the radio-ready polish of its gold-selling 2000 debut, the new release ventures so far away from the band's bluegrass roots that the genre label is no longer relevant, except as a general point of reference. Typical of This Side is the romantic confessional "Speak," which sounds like the folksy side of The Grateful Dead intersecting with the a cappella jazz of The Manhattan Transfer and the alluringly fragile folk of Alison Krauss (the record's producer, as it happens). On "I Should've Known Better," mandolinist Chris Thile provides a woozy string arrangement that obliterates the careful, contrapuntal rhythms, lending drama to the sultry lead vocal of violinist Sara Watkins. The title track applies a straightforward mix of Thile's mandolin, Watkins' fiddle, and her brother Sean's guitar to what, in another arrangement, could pass for sunny power-pop. This Side too often leans on down-tempos, and Nickel Creek's adventurous spirit doesn't always serve them well. Covering Pavement's art-rock anthem "Spit On A Stranger" is amusing, in theory. But while Pavement has a formidable repertoire of catchy, witty songs waiting to be covered by an act without its own vocal or instrumental limitations, Nickel Creek doesn't do much to smooth out the song's raggedness. Still, as a letter of intent, the cover does its job. The bulk of This Side's songs, which were written by Sean Watkins and (mainly) Thile, have the surface of smooth, rootsy folk-pop, under which rests depths of instrumental invention and personal significance. The trio reclaims the bluegrass concept of "homespun," using the genre's even boil and exactitude to lend purity to its smart, moody music.