They are really, really fun. They can be comfortable, I have 5 different windshields for the varying weather and wear goggles on longer trips to keep the wind off my eyes. I hate driving over 50 miles in a car, but 300 in a Slingshot is no problem. Admittedly I'm not stock as delivered from the factory, but you probably won't be either as you figure out what best suits your needs.
Take her for a casual test drive (no fast corners); set this up with a dealer in advance and let them know all you want that day is a test drive, you'll stop back at a later time to discuss the options. Take her out to lunch after. A nice day out for two hours is representative of a nice weekend puttering around in a Slingshot, except you meet far more people driving a Slingshot. It really is a lot of fun to take out with the wife on a weekend.