I'm considering one for the house I've been renovating, but it will have to wait awhile, as I've spent so much already.
I don't have gas service, and I'm afraid of propane, so it will have to run on diesel. If and when I do make the purchase, it will most likely be a Kohler. Cummins and Cat are also good brands. Those three brands are the ones most commonly installed at nursing homes, hospitals, fire stations, phone company switching centers, etc. where reliability is of extreme importance.
I would deal with an authorized distributor of whatever brand I choose; they will put you in touch with a certified installer, and will have a service department - these do require service just as a vehicle does. They will also do a load calculation to determine the correct size for your place.
I have two relatives in Mississippi who have generators. One I think is a 100 KW Kohler for their poultry farm (kept all the chickens alive after Katrina), the other a 60 KW Kohler for a large all-electric home. Both are diesel as they are in rural areas. They have been pleased with them.
One thing you want to make sure of is that is has excellent voltage and frequency regulation. If not, you may ruin sensitive electronics. Also the transfer switch should transfer the load in no more than 10 - 15 seconds after a power outage, and excercise and test should automatically occur at least once every two weeks.
For the total price you will have to add in the installation. Electric codes require that these be installed by a licensed electrician.
Rex (Tolivac) knows quite a bit about these. Perhaps he will comment.