Randy Johnson is one of those players who is potentially a LOT more valuable in games where you can choose a new roster every day (like Rotohog, FantasySportsLive, SnapDraft, and DraftHero) than in traditional fantasy baseball formats. That's because at his age (45) you REALLY don't want to have to count on him for a full, heathy season of baseball. But he's still pretty effective when he is healthy, and is a perfectly acceptable choice for spot starts when his match-up is good. Last year he struck out 8.6 per nine innings and walked only 2.2 per nine innings. That's not out of line with what he's done the past few years, and even assuming a continued decline due to age, I won't hesitate to use him against weak opponents in any of the contests listed above.
You can read more of Alex's thoughts on fantasy baseball at waiverwire.blogspot.com.