Personally I learned a lot about yoga and the classes themselves throughout this summer after attending several times a week for approximately three months. I noticed how different power flow classes were to moksha classes. I feel that the PF ones are a much harder workout since there are so many flows involved! My chateranga is definitely much more stronger now than it was before, and I can flow from one position to another with much more ease. Moksha classes seem to include the balance poses a lot more and are seen as the "intro" class for beginners. Nevertheless I enjoy going to both, it's important to mix it up! I usually just attended the same time slot throughout the week only to find out midway what type of class it was. It was also really great taking classes from so many instructors. Although it might be the same type of class the instructors are what make all the difference and can make the classes completely different and unlike one another.
Yoga definitely is a great workout for strength (holding those poses will definitely help to tone your muscles) and improving ones flexibility.