It's not bad but it's quite small with regard to exhibits and only open to the public on the one day per year as far as I know. To be honest it's probably not worth the trip except in conjunction with the vehicle show. There are huge volumes of classic buses that attend and you can ride between the museum and the seafront where the commercial vehicles are displayed by the Labworth cafe.
We really enjoyed the day despite the somewhat grim weather and there were crowds of people there.