I bought a Mazda6 with the S-plan a few months ago. It's a fixed price, around $150 below invoice I believe, but it varies with the model and options. There is also an "E" (?) plan where you save even more, but I think it's only for Mazda employees and affiliates. Of course once you add in taxes, fees, etc. the price is above invoice but I figure you save 1-2k, and no haggling.
It was a decent experience, follow the rules. If you read them, it says something like...
Step 1: Go to your Mazda dealer
Step 2: Identify yourself as an S-plan member
Step 3: Select your new vehicle from stock
Step 4: Purchase your new vehicle
I drove it first and then told the salesman, but he was cool about it. I knew what I wanted and it was painless. The finance guy may still try to screw you, so do your research and pay attention.
The best thing to do is figure out exactly what you want first and do a web search of inventory, it works pretty good. Then go drive it.
It was worthwhile, and the Mazda6 is a great car.