I traded Volquez/Theriot a couple of weeks ago for a little bit of a hitting upgrade (Glaus/Hardy), and immediately grabbed Guthrie off waivers to replace Volquez. He's been a fantastic upgrade over Volquez in terms of the last 6-7 starts, so it's worked out well so far.
Everyone else here is right, even in my 12-team keeper he's been a free agent most of the year and people have only picked him up for spot-starts. It's true that his strike-out totals aren't gaudy, but he can get you about 4 or 5 per start and is a very savvy pitcher, so you don't worry too much about him getting shellacked in a start. Of course, now that I've said that he probably will, but I agree, he's very underrated.