OK, I'll play, though my views contrast with David's. I have all of the Synchron pianos but no experience with the old Vienna Imperial.
Bosendorfer 130 upright: Beautiful, balanced, warm. Playability and touch are about as good as it gets. Especially well suited for jazz. Not the widest dynamic range, but nice timbre variation across velocities. One of the few VSL pianos where I'm perfectly happy with just a single close mic.
German 1904 upright: More character than the Bosendorfer upright, and it sounds 'thicker'. I think it works well for both soft material and slightly more percussive music. Less versatile, but really lovely - and there's not anything else quite like it from other developers.
Bluthner: A cozy sounding instrument. Sounds its age, for better or worse. Nothing thunderous or majestic about it, but very warm and intimate.
Bosendorfer Imperial: Really well balanced with a nice balance of crispness and warmth. Very well programmed. One I don't play a huge amount but would very much miss if I didn't have it. Not as overpowering as I'd expect for such a large instrument.
Steinway: Beautiful, but a bit clinical for my taste. Requires a lot more velocity tailoring than most of the other pianos, and is a library I didn't really enjoy until adding the mics in the full version.
Bosendorfer 280VC: My second favourite of the big grands. Crystal clear but with with plenty of body. 'Precise' sounding bass notes with a crisp attack I like a lot. Hugely flexible if you dial the timbre back a notch or two, which can transform the tone into something pleasantly woolly. Beautiful for jazz and an instrument I feel really 'connected' to when I play.
Fazioli: The only VSL piano I've tried to return. If it was my first foray into their pianos I'm sure I'd be impressed, but compared to their other pianos I find the sound bland and nondescript. Perfectly pleasant, but without the vibrancy or liveliness of the Bosendorfers, Steinway, or Yamaha. It's also absolutely massive in terms of GB, the fourth pedal is as good as worthless, and unlike some of the other pianos, when I really hammer at it, the loudest velocities are underwhelming.
Yamaha CFX: My absolute favourite. Playability is superb, timbre range is spectacular, combines crystalline highs with rich lows, and has plenty of 'bite' without ever being harsh. It's what I imagine my perfect concert grand would sound like. And that's without upgrading to the full version!