I have been using an RTLSDR to listen to the airwaves for more than 10 years now. I started off listening to VHF/UHF since that is what was supported out of the box on the older dongles. There was plenty of traffic, from local ham stations to BESCOM to analog cordless phones.
But HF is the “real deal”. Getting to hear activity from all around the world is fascinating. Back then I had to build a 100Mhz up-converter to listen to HF.
Coming to the present, the current v3 dongles don’t need an up-converter. It uses direct sampling to achieve this. I had a v3 dongle lying idle for a while now and I decided to build a simple 40m dipole to use with my BITX 40. This needs some impedance matching which will be done later as its main purpose for the time being is for listening.
All the info on buying and setting up one of these is available on the RTLSDR blog.