I don't think that it's a ponzi, it's more of an bubble and an abomination on the original idea behind bitcoin.
The original idea was to keep the bitcoin network spread out because everyone who has a computer, can be involved with mining bitcoin with their existing CPUs. That means that no additional resources need to be wasted to create machinery that supports the bitcoin network and only electricity is used. ASICs ruined the entire concept and actually started to reward those who contribute most to unnecessary wasting.

The idea should be not to build faster and faster hashrate, but to keep the hashrate down and the network spread out as much as possible. The network doesn't gain anything with increased hasrate, no speed, no security, no anything. People are just buying ASICs, so they could get a piece of the bitcoin hype pie, while actually not realizing that their ASICs contribute to absolutely nothing that could be considered relevant.

Scrypt is better, because it uses the existing GPUs and therefor no new hardware has to be created to chase a worthless goal. Primecoin is also very interesting because it can still be run only by CPU. A lot of people who aren't into gaming don't have good GPUs for other uses, but everyone has a decent enough CPU, and so it's easier to keep the network more spread out. But the price isn't currently set by how good is the quality of the coin, but hype is the thing that sets the price. And Bitcoin has almost all the hype.