How difficult is it to install? Can it be done by the average guy? Looks a bit complicated looking at the instructions on line.
It is not complicated, but it can be tricky only because the space you're working in is a bit tight, so it takes some patience to find the spot and get the thing in there.
IMHO, the simplest way is to remove the windshield wipers, the motor, and the tray at the base of the windshield. This provides enough room to access the hole in the block from above. I did mine when I installed my strut tower bar. Removing the wiper hardware & tray isn't very difficult. See the instructions here: http://priuschat.com/forums/gen-iii-2010-prius-accessories-modifications/88655-tanabe-sustec-strut-tower-bar-under-brace-diy-installation-pix.html There is one trick: a wire harness behind the engine needs to be moved. See the instructions here: http://techno-fandom.org/~hobbit/cars/bh3/
I can say I felt the same way, but I've worked my way up from smaller things (like oil changes) to bigger jobs like suspension mods. Look at the instructions and think about it - your confidence may grow with time.
I am no master with tools and I did mine. I would suggest giving yourself 3 or 4 hours and buy all the tools first. A very long extension to remove the wire harness bolt for example. All you have to do is feel around for the hole and you should find that no problem too. It's no big deal if you allow yourself plenty of time. Just make sure to follow those instructions exactly and you'll do fine.
Would it be difficult for someone who is experienced? My husband is a Saturn/GM technician, but doesn't have any experience with the Prius.
Mare, if your husband is a technician/mechanic I don't think it would be a problem for him. I don't think it's all that complicated or I wouldn't even consider doing it myself. I'm sure others will chime in on this for you.
With his experience and the info on this forum about installing the block heater, he shouldn't have any big problems. There will be some learning curve with any new vehicle. Have him look at the links I gave above and then look at the engine compartment. Elsewhere on this forum, you or he can find a complete set of maintenance documentation for the 2010 Prius. That should help overall. The neat thing about the Prius is that it needs significantly less maintenance than most any other car.