Hey Oren. A friend of mine is building a new house and is using these guys - Everlight Electric. He's almost as picky as I am so I trust his recommendation. :) Post back here if you end up doing more research. Thinking of putting one in myself...
A good friend of mine had theirs installed by Premier Mechanical & Electric, and he's very happy with it. Sounds like it seamlessly turns on if the power goes out.
Assuming this is for your home - I installed one a few years back. Purchased a Generac 20,000 KW unit and had it installed by local electrician. Make sure to have your gas main sized as you will likely have to enlarge the main going into your house as they require a lot. Otherwise was pretty straight forward. Took less than a day.