I generally don't like any of the freeware reverbs apart from SIR (which is pro quality, depending on what .wavs you throw into it) Reverb can make or break a track, and bad reverb is just bad with no redeeming features on the whole IMO.
However, I'll make an exception to Pianoverb. It is in no way designed to be a general purpose room or hall reverb, pretends to emulate no real spaces and for what it is designed to do, does it very well. It's designed to emulate a metallic string based reverb - or more of a string resonance, should I say. It does this very well, and although I don't often use it, it usually surprises me at how effective this can be. If you use it to make a drumkit sound like it's played in a drumbooth or studio, then you'll fail miserably - but as should be obvious from the name and design, if you use it on piano, clav or harpsichord patterns, then it stands out. Because it doesn't have the usual reverb controls, and it can be specifically tuned to a metallic resonance it won't swamp your mix with nasty artificial reverbs aka Freeverb, Ambience etc. But it will give a sense of space and almost realism to dry keyboard sounds. I've also found it useful for guitars as a kind of spring reverb when used in a chain of effects - it very much suits the cheap and nasty kind of cabinet reverb you might often want to use with guitar tracks (or on synth tracks that you want to sound guitar-ish).
It has a definite flavour and will do what the blurb suggests it'll do - use it outside of this scope and it's your own fault, not PSP's.
The GUI is pretty, distinctive, easy to use and obvious. It's also fairly small compared to some so it doesn't take up too much screen space, but the design of large knobs means it's also in no way fiddly to use. One of the best GUIs IMO - alongside things like the Classic series from Kjaerhuis (or however it's spelt).
Features - various tuning parameters, dry/wet mix, decay etc and a useful bypass button (I wish all plugins had a bypass button - top marks PSP!)
One-trick pony? - sort of, but it's meant to be, so nobody can complain about it not sounding like a Lexicon - it was never meant to!
It's free, it's of the usual high standard set by PSP and of the usual distinctive character from them. Not a must have if you're purely electronic or synth based - but if you ever use piano, harps, clavis, and even guitars then you should do yourself a favour and try this one out as an alternative to the usual crappy freebie reverbs that almost never deliver (apart from SIR).
And it's bright yellow - how many other plugins are that pretty?