You probably will get opinions of both sides of the fence about this topic.
For me, my experience with the puppy classes at Petsmart was a pleasant one. We had 8 weeks of lessons and really did learn alot. We really enjoyed it and it did help Ginger with her socialization and basic commands. Our class was a small one (6 pups total) and luckily the dogs were mostly small breeds like Ginger. There was one Boxer pup and a mix but they were leashed at all times and there was no problems whatsoever. All pups were super friendly... no growls or anything like that. Just alot of happy tail wagging and sniffing.
We were in the "Puppy Class". All of the dogs were young puppies so that did make a difference, I'm sure. If I had it to do again, I would. It was alot of fun!
I think it also depends on the individual trainer. Our trainer had been training for many years and really did a good job. I went in the store prior to signing up to meet her and kinda check her out. I wanted to see what she was all about - see how knowledgable she was. I could tell from the initial meeting that she would do a good job and she did.
I even asked her what other dogs were signed up because I was worried about the size thing. She took out her list and shared that with me. It all ended working out just great!