While only unencrypted systems can be monitored security flaws in portable radios used by federal law enforcement agents (FBI, DEA, CBP or Secret Service) allowed researchers to intercept sensitive information, according to a research study. The team included University of Pennsylvania Professor Matt Blaze, Sandy Clark, Travis Goodspeed, Perry Metzger, Zachary Wasserman, and Kevin Xu discovered a serious practical problem that can be exploited easily today against fielded P25 systems: "a significant fraction of sensitive traffic that users believe is encrypted is actually being sent in the clear."
Recently YouTube user Rob Fissel has uploaded a video showing a comparison between a cheap RTL-SDR receiver using the GNU-Radio APCO-25 software decoder (OP25 project) and a old school Uniden BCD996T.
While both setups can used to decode P25 Phase 1 LSM signal OP25 does a better job at decoding a weak signal and producing voice, whereas the Uniden BCD996T doesn’t even manage to hear the control channel.
Using cheap RTL-SDR dongles with software like DSD+ can also be used, but OP25 can decode more systems and show you low level information, even decrypt encrypted traffic when the key is known and log traffic to disk for later analysis.
Even if there is no P25 signals around you a dongle sdr can be used to receive analog signals from walky talkies used by business (MURS/GMRS) to air traffic controllers.