1.) It seems to listen to messages from the cell towers rather than listening directly to phones. This means you are likely to catch IMSIs from a very wide area (i.e., every phone that connects to the cell tower you're listening to), which means you don't get much information about "who was in the local vicinity". This probably also means you can't tell the phone's signal strength, as the received signal strength is that of the tower not the phone (unless the tower modulates its transmit power based on how poor the phone's signal is?).
2.) It can only listen to one frequency at a time. This means you need N RTL-SDRs if you want to listen to phones connecting to N cell towers. (But the RTL-SDR does have about 3MHz bandwidth, so in theory it should be possible to listen to multiple cell towers simultaneously).
3.) It can only see IMSIs when a phone first connects. I don't think the IMSI is sent when making a call or transferring data.
4.) It only works on GSM, i.e 2G. Hardly anyone uses that these days. (I don't know if a similar passive approach would work on 3G and 4G, it may be that only software is required in order to support more than 2G).
The benefits of this approach compared to other things I've seen are that it is totally passive, so even a savvy target is unlikely to be able to detect that his IMSI has been caught, and it is extremely cheap to carry out.