I love Ultra Focus! I have read some good reviews of it in "Computer Music" and "Sound on Sound", tried the demo , loved it, and decided that it was too expensive (almost 400 Euros). It has become cheaper a few months ago, I paid roughly 200 Euros for it. Great!
The GUI has a very solid feel to it and is nicely laid out, no sub-menus, you can see everything you need right away.
The sound is absolutely great, they sure used some great equipment to record those synths, and the patches sound solid, deep, in-your-face, and great to work with. The highs and lows are there, it's punchy and precise. I LOVE it! They covered every kind of synthesis method and chose the greatest and most influential and recognizable synths for this product. The effects section sounds superb and adds even more depth to the sound. Everything feels "high class". Average CPU load (with FX), not easy on the machine but not too much either. Absolutely usable for me.
Since it's a sample player with quite a few editing options, you can change a patch a lot, but the sample of course stays the same, so possibilities are limited compared to a proper synthesizer. But then again, where do you get so many great sounds in such great quality?
There are a few other products like "Sonik Synth 2" (which I own too) that claim to be a comprehensive synth sample library, but to my ears none of them sound so powerful and mighty as Ultra Focus.
Presets are great and more than enough, value for money is now unbeatable, no problem with stability so far, and customer support has always been friendly to me.