I've tried teaching myself Polish several times and it is an impossibly complex language. I have about half a dozen books of this type and this is the one I keep going back to. Its explanations are better than all the others (even if sometimes the example sentences are extremely obscure) and I got to about chapter 6 before getting completely lost. The next best one is 'Polish in 4 weeks' which has more accessible conversations but not such good explanations of the grammar.
I haven't tried using this book with a group and think this might be useful and I certainly never practice enough or do all of the exercises so it is my fault that I still cannot speak to my inlaws!
I am still looking for the best teach yourself Polish book (Teach Yourself Polish isn't it!) but this is the best one I've come across so far.