And you should also consider whether it will really save you time. If you want to use the theme exactly as it's intended, I think it can. Otherwise, it's tricky. I highly recommend using themes from people you trust and whose code you know.
A reader of Paul Boag's site asks if it's okay to use a pre-built WordPress theme for a client site. Two people share pros and cons. The much shorter answer is, yes. It's okay. But, you should absolutely disclose it.