I use purigen and I like it, but it isn't really comparable to ammonia chips (ie: Zeolite).
Purigen is more like a selective version of activated carbon. It ends up reducing ammonia, but not directly. It removes organic waste, which ends up preventing ammonia from forming from that source in the first place. It does nothing for the ammonia portion of fish waste, only the longer-chain organics that it contains.
Since Purigen is selective, it doesn't impact your fertilizers as much, and lasts a lot longer. It can still affect medicines that are based on organics. (I theorize that kordon ich attack may be affected, since it is a plant extract.).
It may help protect you from ammonia spikes caused by rotting plant leaves, but it is really more used for its "water polishing" effect. It makes water extraordinarily clear, removing organics that can give it a dingy tint or make it just a tad cloudy. I have a mild GDA outbreak at the moment, but before that purigen made the water so clear it was hard to see. It looked more like fish floating behind glass, which was pretty cool.
Zeolite - removes ammonia
Purigen - removes nitrogen containing organics, possibly including a few medicines. Indirectly controls ammonia, nitrite, and nitrate as a result.
Activated carbon - removes organic waste, chlorine (slowly), and a wide variety of other things (including several medicines, minerals, metals, and fertilizers).