I'm an observer for the Bolton group. The first thing you need to understand is that the groups are run as charitys, and as such the members give up their time for free. Because of this each group does things slightly differently, and the process I describe is only valid for some:
The IAM Skill For Life package costs £85 (£75 if you are 25 or under). This covers a years membership of the IAM, your test fee, membership of your local group, and a number of textbooks. Once you purchase the package, you will be registered with your local group and they will contact you directly to arrange your start date etc. The practical driving sessions are free.
At Bolton, we run a structured course that lasts a minimum of 8 weeks but more usually takes 12 weeks to complete. The sessions are run on Sunday morning, and consist of you driving your own car with an observer (different person each week) sitting in the passenger seat to provide guidance on your driving, the sessions typically last for two hours. The first four weeks of the course are used to teach you the fundamentals of the system of car control, steering, observation, road positioning etc and then you are taken for a demo drive on the 5th week, where an observer shows you how it all fits together. After this you are just on genreal practice drives until week 8 where you take a mock test. If you pass, you'll be put forward for the full advanced test.
The only pre-requisite is that you hold a full UK license and have not been convicted of any serious driving offences (dangerous driving etc).
Is it worth it? Yes, definitely. If you feel that driving is something you could improve at and are willing to learn then you will get a lot out of it. However, don't do it for the insurance discounts as they don't amount to much (5% if you're lucky) - a much better benefit is the 25% off AA membership.
The main thing that the IAM try to teach is smooth, safe driving whilst making decent progress, to be honest the training on observation alone is worth the fee.
The only thing i'd warn you about is that some groups are a little "backwards" - they tend to be looked after by archetypal "string-back gloves and flat cap" drivers. Find out how your local group does things before you sign up, if you don't like the sound of it there may a be a group a little further afield that will suit you more.