User Interface: The user interface is as good looking as it needs to be. It's pretty enough, but it doesn't have the cosmetic shine of some other plugins (not that it matters). It is very easy to navigate, however. The controls are well spaced and well labelled, organised into sensible groups. Everything has descriptive tooltips, so if you're unsure of what something does, simply point your mouse and hover and all will be revealed. When I first used the plugin, I didn't bother with looking at the documentation, and within minutes I felt at home and never once struggled to do what I needed to do.
Sound: I can't really fault how this plugin sounds. It's definitely the best sounding piano plugin I own, and sounds as nice as the nicest piano sample libraries, but with infinitely more flexibility.
Features: You would think 'a piano is a piano - what more can it do?' but that is not the case with Ruby Piano3D. Almost everything about the sound is customisable. You can tweak the positioning of the microphones, choose how open the lid of the piano is, and so much more. You can even change the effect of the hammer on the strings to get away from a standard piano sound into something more violin-like. Another thing that I just love about Ruby Piano3D is its size. It's three hundred megabytes - not a ridiculous size by today's plugin standards, and yet its a fraction of the size of some piano libraries (with the added bonus of being way more customizable and flexible than purely sample-based efforts).
Documentation: There is a good amount of information on the website about the features of the plugin, and there is an extremely comprehensive PDF manual available (I was sent it by email) which breaks down every feature and offers some great information about mic types, the 3D sound, etc. Plenty of screenshots too. As I mentioned before you really don't need any instructions - you can dive in with no trouble. It is just about as straight-forward as you could hope a plugin to be.
Presets: To be honest, it's not the sort of plugin that could make great use of presets as it is so easy to just dive in and create your own by experiementation. However, a small handful of presets are supplied.
Customer Support: They have a contact form and email address on the website, both are easy to find. I have spoken to customer service via email a couple of times, both asking questions before and after getting the plugin. Each time I heard back within 2 days. They have a concise FAQ on the site explaining things like license transfers, and the amount of machines you can install on.
Value For Money: At $129 it sits directly between budget piano and the high-end pianos that are available. It still offers fantastic sound quality and flexibility for the money, and at a slightly cheaper price point it would be a killer deal, an absolute no-brainer for anyone looking for a very comprehensive piano at an affordable price. I have spent money on a couple of piano plugins/libraries that have disappointed me and let me down, and I know I would have been better to have saved that money and put it towards Ruby Piano3D.
Stability: I tested Ruby Piano3D in standalone mode, and the VST version in Renoise and Reaper. I had no problems with any of them, apart from in Reaper when I used Reaper's MIDI humanization plugin which caused occasional crackling (but I could only get this behaviour when feeding the piano with Reaper's humanization plugin, so I don't think I can blame this on Ruby Piano3D). I am running a dual-core Vista laptop with 4GB ram.