The Generacs have automatic load switching. Their biggest reasonably priced consumer model is 24k or 26k, that's enough for a 5000+ square foot home with all circuits piped in.
The thing I like about the Generac (and similar) is it forces you to get an excessively large propane tank that you *should* have anyway to do deal with acts of god that might ruin all your perishable foods. Total cost for the biggest Generac installed is roughly $14k. The first time a storm takes your power down for 5 days, you'll probably save $5k in groceries that would otherwise perish.
I agree, its not the right investment for someone that will move in 3-5 years, you'll get nothing for the Generac in the sale of your home. It's definitely something to install in your forever home for your pleasure, not the next buyer's.