If you like Pinor Noir - try Davis Bynum! Small winery, great service and awesome wine!
Hop Kiln is nice for its' scene, but I find the wine just ok.
RoShamBo has moved down the street (couldn't afford the rent anymore).
- Wine Tasting
While you are visiting Guerneville, you may want to take advantage to the proximity to the many wonderful Russian River Wineries. Many of these are small, family run vintners, and as such, are far preferrable to visit that the big corporate entities that are found in the more popular Napa Valley.
Russian River Valley Wineroads Map, offers an excellent view of the wineries and information about touring and purchasing. Links to most of the wineries are embedded in the map on the website.