PR's basically just substitute for a port, except PR's are a lot more practical due to how much more air they can move (ports have problems with compression at loud volumes - the way around this is to have a port with a wide diameter, but then you have to make the port longer to compensate, which becomes very impractical very quickly).
The reason most PR'd subs sound bad is 2 fold - 1, they are not tuned low enough, with leads to group delay that is pushing up in to the audible range. 2. People can see them as an excuse to use a smaller box than they would otherwise use, which is a bad idea if you want best transient response.
Sealed subs do have the possiblity of giving the best transient response, because the Q can be set to .5 which is considered critically damped, but the drawback of sealed subs is that they start rolling off pretty high, and then depend on room gain to provide low frequency boost, which does not always happen (and never happens smoothly, with any sub - I recommend using an EQ on any subwoofer for best results in the sweet spot).
Having said that, I still have not heard a better, more musical sub than the VMPS subs. On paper, the PR based subs should not sound as good as a quality sealed sub, but in my direct personal experience, the VMPS subs are quite a bit better than the sealed subs I've heard (commercial and DIY sealed subs, that is).