Anyone who believes that FirstNet and other "public safety broadband" is about replacing the voice communications we first responders use today doesn't have an understanding of what PSB is to be used for.
Skimming through this thread, I see people saying 'yeah, but what does [it] do that we've been saying 'I wish we could have ___' about in the past?'. Don't get caught in that mindset. Some of the benefits are things you'd never have thought possible in the past. One of the demos I saw a number of years ago (5-7 IIRC) - admittedly not FirstNet but the equivalent in western Canada - involved getting a video clip from a convenience store surveillance camera, of the bad guy, sent to the MDT and smartphones of the responding police cruisers. Yes, there would have to be a number of things line up right for that to work - the store's surveillance system would have to be able to get a clip out to the police dispatch center, is the big one - but the role of PSB in that situation is to provide a conduit for the large amounts of data that can be shared to the field crews (and vice versa - imagine attaching a video-recorded witness statement or 360° size-up of a burning structure to the event record). Other uses, as has been said above, might include preplans/floor plans, inspection records or premise history; carrying MDT data (that is currently on either owned radio channels or public mobile phone/cellular networks); even downlinks from video feeds, either to the on-scene command post or to the boardroom in headquarters for the honchos to keep watch over the situation.
PSB is just a big pipe that is dedicated to us public safety users only, for the purposes of moving data that we may or may not have wanted to move previously.
In the future, we might use a little bit of that dedicated-to-us pipe to move our voice comms across, rather than using public airwaves. But there are very few that are using it for that purpose today.