My brother and I decided to take the full-day Napa Valley Winery Tour (the 4 Winery tour with SF Bay Ferry ride) back in October, 2011.
The shuttle bus operator was friendly enough, and the Wineries were beautiful. Wish we could have spent more time at them.
The only problem was that we missed the ferry ride on the way back. I think the ferry company changed the schedule, and we just missed it. However, the company did do the right thing and gave us a partial refund.
As I said, it's a nice tour (and reasonably priced) if you're looking for an all day guided trip.
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