I did 15 hours in an Elite FNPT 2 for my IR. I already held an MEP rating beforehand, which made the sim slightly more difficult at first - people are right when they say there is a large difference between the sim and the real thing and the sim is harder to fly.
That said, I think it was very useful time. I got a chance to work on some of the basics in detail, and the chance to look at your 'pattern' after the flight is also useful if debriefed. I would agree that 15-20 hours is about the most I'd want to do though. After that, I think you'd get too used to the sim.
I haven't had a go at an FNPT I, but I think it's unfair to say the II is only a marketing tool. Sure, the basic skills can be taught well in either type, but practicing the instruments to visual phase on finals, circling approaches etc is quite useful and reasonably realistic on the II. If, as in my case, you test from a different filed to where you train, it also has its uses as an aid to learning all the taxi routes!
Worth mentioning also that not all schools will use the sim as a money-making machine. Mine gave us unlimited free use of the sim and we were encouraged to go and look at specific debrief points as well as using it for general procedural practice.