Grohl wrote most of the riffs, with 11 singers -- from Lemmy, Lee Dorrian, and Wino to Tom G. Warrior, Cronos, and King Diamond -- contributing lyrics. "Shake Your Blood," featuring Lemmy, and Wino's "The Emerald Law" are hard-rocking standouts. Probot will fortify Grohl's reputation as a drummer, too -- he plays ferociously, in a variety of styles. But it's that variety that's slightly problematic. Since each song on Probot is geared toward its vocalist, the album often sounds like a compilation, rather than a cohesive effort. If this disc is some folks' introduction to these steel-studded throats, that's glad tidings indeed.