When I got my first rtlsdr dongle I spent hours just surfing the frequencies looking for interesting signals and trying to decide them. It's a great hobby!
SDR has opened up a new frontier for me, it gives me the same excitement I got when I discovered BBSs and then the internet.
I was lucky enough to have a project come up last year that required gsm/4G and GPS simulation. I got budget to buy a bladerf and a nice SBC and built a box that could simulate a GSM, 3G/4G basestation and simulate GPS.