I had a RadarBox for a year. It's a great device. There's a decent number of ADS-B planes to see. But even just watching the mode-S returns you can get a feel for the traffic in the area. You get altitude info with mode-s hits.
Out of the box RadarBox is easy to set up and get running. You get a year of free shared data. After that you have to pay. I wasn't a big fan of that.
I wanted to interface with Planeplotter software and be able to do multilateration (basically be able to use multiple receivers and the software to compute the location of non-ADS-B aircraft) the RadarBox doesn't give enough data to Planeplotter to do this, so about a year ago I got a Kinetic Avionics SBS-3. It's quite pricey, but I love the features: 2 SDR receivers that can be split into 6 receive channels,internal ACARS decoding, data stream the allows multilateration, and other features. It's definitely the top of the line, but worth saving up for.
If you wanted to go a lesser-cost route, you can also use the RTLSDR dongles to receive the ADS-B and mode-S signals and feed Planeplotter. I haven't tried it, so I don't know how hard it would be to set up. But quite a few seem to be using them with good success.
Having your own virtual radar is a lot of fun if you're interested in aviation and listening to ATC. It adds a whole new dimension to the hobby.