How many people are in your group? Do you need private offices or are you ok working in an open environment with use of a conference room when necessary? How long do you need it for?
I'm personally at Nextspace (http://www.nextspace.us) and really love it. Its on 2nd and Market. There are a few other co-working spaces that are much better than Regus in the area:
- http://www.sandboxsuites.com/ (multiple locations in SF)
- http://www.rocket-space.com/
- http://wework.com/ (not sure if they're opened yet)
- http://bayarea.the-hub.net/public/space__Hub%20SoMa.html
Also check out (http://www.loosecubes.com/) for companies renting out extra space in their office.
Email me if you have questions about NextSpace.