I've been going to shows at Larz Anderson for over a decade now. British car day is hit or miss. In the past there have been McLaren F1's and all sorts of Astons and Jags, it really depends on the weather. Typically there are some great classics as well (Jag 120's, Astons from the 60's, old Lotus'...etc).
Both German Car Day and Porschefest are typically good. Italian Car Day (Tutto Italiano) is a great show as well.
The museum itself has some interesting cars and overall its a great venue for a car show. I'd say it's definitely worth it.