How popular are prime coins and would what would you do with them besides exchange them for btc?
They are very popular, very new, and there is nothing you can do with it yet. It's very much a developers toy atm. It opens up a new direction where all the power used for minting coins is actually useful and it uses a 'real'-world rarity, namely finding new primes. This will continuously be harder similar to actual materials, rather then something thats rarer merely by the definition of a pointless formula.
Since they work in conjunction with GPU miners, the network already has a huge amount of hashing power when comparing it to the value of the coin. But as it is, it's in addition to what you make on your GPU miner, so its a plus, since your machines are already mining anyway.
