Flying Vikings has itermidiate maintence, and Cal Airways is on the verge of going out of biz. Aclan Pro Air, is a good learning enviroment, has good equipment, and is growing. I do not know anything about Diamond. Ask away about HWD as I do a bunch of teaching there and other flying.
I recommend Chris Grcevich, operating out of KCCR's Sterling Aviation - he has access to several different aircraft ranging from a piper cherokee (archer II), C150, C172, C180, Cirrus SR-22, and probably a few others by now.
I flew with him in highschool and he's got tons of hours, lots of experience and the patience and understanding we all would hope for when trying to get through certification.
His flight dynamics are also very progressive. I spent most of my time over Sonoma County (SW of travis AFB) in "the practice area" with no other planes, lots of fun stuff, plenty of room to do your steep turns, engine failure, stalls, wing-overs, lazy 8's and yank&bank along the river with no worries in the world what so ever.
Sorry i didn't specify, but I'm looking to flight instruct somewhere out there not train. I've heard nothing but good things about Aclan Pro Air, but I would prefer something in San Carlos or Palo Alto.
I got my PPL at Flying Vikings, but that was a long time ago. Still the same ownership now I think. They were okay.
Cal Airways I was not impressed with- the owner seemed erratic and acted like I was wasting his time (even though I was a potential paying customer just asking a few questions) Also, instructors have to pay a "desk fee" to teach there (that is what one guy told me). The school is mostly Indian students now.
Aclan Pro I have also heard good things about, but have not checked them out personally.
If you are looking at PAO I would also recommend Shoreline Flying Club.