Let's get the eval out of the way up front: Pianissimo sounds great. But there are a lot of great-sounding software piano VSTi's (short for Virtual Studio Technology instrument, a software synthesizer plug-in standard from Steinberg) out there. Some of them with way more than the 80 megabytes of samples that come with Pianissimo. They also generally have a lot more in the way of a cash requirement.
I plugged the $69 Pianissimo into several of my songs that use piano, and while it doesn't have the bite of some I've tried, it has a pleasing quality that I found very musical--and very addictive. Pianissimo allows you to adjust the level of the hammer attack as well as the tone via a three-band EQ, a sympathetic resonance control, and simulated piano cover positions (closed, half-open, etc.). There's a default MIDI file that auto-loads so you can adjust the sound while it plays. Nice touch.
Pianissimo also comes with a nice assortment of reverbs plus chorus, though most users will cover those with their favorite plug-ins. Maybe not; the FX sounded pretty okay for the ilk. The integrated FX are also handy for spicing up the sound of the standalone version of the program.
Said standalone version of Pianissimo also loads/plays and records/saves MIDI files--a super handy capability if you want to quickly save an idea, study practice sessions to look for flaws in your technique, or simply record a track without booting into full-fledge sequencer. For those listeners of yours without a high-quality piano module, you can also mix down to wave, MP3, WMA or OGG formats.
Taken as a whole, this is super nice little piano program/plug-in and its sound has really grown on me. I'm actually using it on a couple of my tracks, which is the highest compliment I can pay it.