I've done this particular tour twice now, and I'm looking for excuses to make a third trip. Hunt was our tour guide both times, and he's a blast. Super knowledgeable, patient with new Segwayers, and though he's done the tour probably a billion and a half times, he thoroughly enjoys what he does.
If you've never ridden a Segway before, fear not, it's easy. Whether you're in shape, out of shape, young, old, what have you, you can do this. And it's FUN.
You'll generally go to two out of three wineries they work with, and the Vella cheese factory. All three wineries (Ravenswood, Buena Vista, and Gundlach Bundschu) have a unique history and beautiful views. Your Segway has a bag attached for your belongings and any bottles of wine you'd like to purchase, and Hunt has extra storage space on his Segway as well.
I'd highly recommend checking this tour out. Yeah, there's the napa wine train and bus tours, but this is the most unique way to go, and the most fun.
Look for 'white lightning', the white segway :) She's a good old gal...