I just set mine up Friday night. Haha.
It depends on the height of the antenna on each boat. When I'm running with similar boats(antenna heights) 10-12 miles can't be counted on.
Unlike routes, maps, radar, sonar, and track data, Garmin doesn't share MMSI/Other Vessel data on the network. I program 2 MFD's and the VHF with vessel data. I can keep up with it now, but at some point I'm sure that will get away from me. I've programmed my own info into my friend's VHF's. I wish there was a way to import/export, but no such luck. (my SH portable can, but even SH fixed-mounts can't!)
Wha huh? Did someone just say, "Rollercoastr, go get your soapbox"?
I've been on a mission for the past few years to get as many boats as possible to fully-enable DSC. This summer I decided to try to get every boat on my pier configured. I know that's not realistic, but I've had more success than I expected.
Position Requests are kinda cool. I think for me they're about 3rd in usefulness as far as DSC features go, but considering how important Distress Calls and Selective Calling are, that's saying something!
As Creekwood mentioned, the directory issue is probably the biggest impediment to the PosReq feature. I keep a Google Sheet with the vessel and MMSI info of the boats I've helped configure. I then take that data and program it into my Garmins. All of them.
Regarding Position Requests: I certainly haven't seen every VHF radio on the market, but the ones I have worked with default the reply to a PosReq to "prompt". In other words, if someone requests your position, you have to tell/allow your radio to respond. My Garmin has PosReq settings including "automatically reply", "Manually reply" (prompt) and ignore. I keep mine on auto.
A good friend of mine recently bought a new to him, old boat. I didn't give him a choice about upgrading his VHF and connecting it to his plotter. Our first venture out was a raft-up - a great day. When time came to weigh anchors, I idled nearby to make sure he didn't have any issues. He got on-plane, so I hit my own throttles and cranked-up the stereo and headed even further into the Verizon dead zone, enjoying the sunset cruise home. Minutes later, my VHF starting ringing. I looked back to find him stopped. He had lost his port engine. He said he had tried to text me and hail me on VHF, but I don't know what channel I was even monitoring or if I had the volume high enough. DSC worked though!
Since then I send him PosReqs when he goes out without me. I can make sure he's making headway and see exactly when he's heading back to the dock.
Regarding those DSC calls: yes, the digital signal travels further than an analog call. That's helpful when sending a Distress, making a selective call or sending a PosReq. It doesn't however help with conversations - that's still analog and non-private. When placing a DSC call, the digital part is the ringing. Only the intended recipient's VHF "rings", and that's sent digitally over ch 70. When the recipient acknowledges the call, the radios negotiate to choose a regular working channel such as 68, 72 etc. and from that point forward the call is a plain ol' public analog conversation. (my friend's SH likes to choose Ch 6, which always causes me to double-take)
Regarding test calls: this one surprised me. I too had spotty luck with test calls to the USCG test MMSI 003669999. I suspected that I had some antenna issues, but I've since learned that 003669999 is not automated! Someone at the USCG has to manually respond to provide a response! Ugh. It was disappointing, but I probably shouldn't have been surprised.
Regarding Distress Calls: you guessed, more govmint ineptitude. I assumed that after registering my info with an agency who provided me with an MMSI and then shared that data with the FCC who shares that with first responders such as police and CG, that in a Distress event, that data is automatically available to those first responders. Typing it out, it even sounds naive! Sure enough, it's apparently rare that the personal and vessel info we provide in order to obtain our MMSI's is available to first responders. That's discouraging, but it also under-scores the critical importance of providing position data to your VHF radio, as that's the most important data in a distress situation.
