It's the first altcoin that is using the hashing power to calculate something useful. And it's still the only one that does this. Primes are (like already said) very important in cryptography and of course can be useful in other areas as-well. People like the idea of using the hashing power to create something useful. I'm not an expert on prime calculation but the used ways to calculate primes seems to be 3 of the few to go at least for now. There will likely be hard-forks necessary as soon as newer and better prime calculation methods come along and make the used obsolete. That isn't a big problem with an active community behind it and will not have to be done often if even once. Sooner or later the development of ASIC's will be worth it and calculating primes will get to the next level. I think XPM will stand around for a long time and will very likely increase in value over time (not necessarily according to BTC but for sure in relation to the USD).

My current full list of preferred cryptocoins: 1. BTC, 2. LTC, 3. XPM