When I first got my guinea pigs, I wanted to take them to the vet to make sure they were sexed correctly at the pet store (all I need is 5 million more guinea pigs) and that they didn't have fleas or any diseases.
I ended up at Carolyn McGuire's. The vet was able to reassure me that I had indeed ended up with two females, and they were nice and healthy.
As for actual problems...when I later had problems with their health, I was told to take them to a local vet hospital that specialized in exotics (several other vets told me the same thing)
Overall, at least in my case, they were great for general health of my guinea pigs, but not able to help with problems.