I've used Nonpareil quite a lot. I've also donated to Eric's development fund.
This simulator is nicer. It may not be as cool - Nonpareil being a machine level simulator, and part of a mind bogglingly huge subset of simulators of old HP calcs - but this simulator is nicer. The 15C skin is nicer. The integration of preloaded programs is nicer. The ability to save and restore memory and user programs is nicer.
I don't think that detracts from Nonpareil at all. Eric's goal seems to be to simulate the widest range of vintage HP calculators (41C and before only, so far) possible. He has this nice architecture for doing that. His code is GPL'd, which is not only cool, but generous. However his user interface is somewhat basic. AFAIK, there's no easy way to load user code programs into any of the simulations from outside, to take one example. Mind you, I'm NOT bitching about that. (If I were, one stock - and correct - response would be "send in a patch.") But I think there is room in this world for more than one implementation of Voyager, and many other HP calculators. I also don't mind paying $20.00 for what looks like a good implementation, particularly if it runs on my desktop platform of choice.
Speaking of closed source programs running on Linux, have you checked out the beta of Google Earth 2.0? Runs on FC5!