Insofar as POW works in general... you cannot stop ASICs, period.
It simply is impossible.
With SHA256d, the availability of GPUs and FPGAs made it so the leap from CPUs to ASICs was bearable, and did not compromise Bitcoin's security.
Without the step of GPUs and FPGAs, this leap is almost certainly to be deadly to any cryptocurrency.
In the meantime, before ASICs make sense financially, you have a system that gives criminals (computer crackers) the ultimate authority.
So, to conclude, GPU-resistant is very bad for proof-of-work systems.
I disagree, it's not the point of stopping ASICs, they are the best thing happened to Bitcoin because they are ultimate protection from 51% attack. But there is no reason to develop ASICs for alter coins, regular people need access to cryptocoins for large adoption. People love lottery. You can't have global economy without very large adoption of currency, so Primecoin may actually be very, very good thing. There is a reason for such avalanche adoption comparing to other altcoins.
I didn't claim that XPM is GPU-resistant, to my limited knowledge it should be, but it may not be the fact. Certainly will be for some time, giving it a chance to be really broadly adopted.