I think you should not expect the children can learn much more acadamic knowledge in playgroups but just let them feel about what is school life, how to communicate with others, sharing, discipline & etc....
Highly recommended! My daughter started her playgroup since 11.5months.
A great time for her and me too. You can learn how to play with her. Sometimes you cannot think of simple and easy games but the playgroup can give you some ideas.
My daughter played with finger paint when she's 1 year old. Now she played with paint in the tub almost every night. Very messy but lots of fun.