Overall fantastic way to spend a day and learn about Napa area, wine-making, and get around. Especially if you are a first time Napa visitor (I was) this is a great way to orient yourself to wine country.
I always look at negative reviews before booking a tour, and I'll say each one I read turned out to be completely false, or the reviewer didn't do his/her homework.
Yes, you need to purchase your tastings at each tour yourself, but this is clearly indicated in the sign up and if you've done any research ahead of time you'll find it is standard for any Napa tour. Yes, your tour may be subject to change. Good luck finding one that is not as the touring company can't control conflicts with other private events, closings, acts of god.
You are paying for:
a) transportation (look at the cost of Ubering and compare, as well as organizing any private tastings)
b) catered lunch
c) outstanding tour guide who will teach you about Napa regions, winemaking. Leif was funny, knowledgeable, and patient.
d) A fun experience with a group
e) Likely at least one winery tour behind the scenes. We toured the indoor facility and got to pluck grapes from the vine to sample at WhiteHall
As an added bonus, if you are subject to motion sickness (especially after drinking) the open-air trolley is a great way around it.
It was everything I wanted when I signed up.