They are just a hybrid of conventional liquid electrolytics and polymer solid capacitors, so they're not exactly like a conventional "wet" capacitor and not exactly like a regular solid polymer capacitor. The reason it's hybrid is because solid polymer capacitors aren't available in higher voltage ranges. If you don't need really high voltages, a regular solid polymer would be better. I don't now how they compare to tantalum, but the solid polymer ones have been used in higher quality PC motherboards since around 2007.