I recently bought the Locale Google Voice plugin so that I could set up a Tasker profile so that GV would only ring where I needed it as I had a problem with a personal call going into my work VM on a holiday.
Anyway, I was able to use the plugin to easily set my GV's Google Talk to forward without having to go to the web. That was pure luck.
I wanted to have Sipdroid to make/receive calls over wifi at home, but was disappointed in the call quality. For some reason, I thought VOIP would be as good or better than 3g, but I was mistaken.
But the quality is great if I were to use it internationally since it would be "free"
Consider getting the Locale plugin and GV Callback if you already own Tasker or Locale to further customize how you use Sipdroid/GV.