The PVM-21, as a shot recorder, works great. It WILL record in any conditions, including complete darkness. It is reliable and fast to set up, and it does have a nice, big "Sweet spot". It is our preferred testing unit (and we have access to Oehler, PACT, CED, Shooting Chrony, RCBS). It is outstanding in its capability to record the shot and show a speed.
That said, 90% of the users are essentially using their PVMs manually -- the base unit shows shot number and velocity -- and the users are writing these numbers down with pencil and paper, and then doing their own ES and SD calculations with hand calculators.
Why are people having to write down the numbers? The reason is the unreliable software. The PVM is designed to hook into a laptop computer. Unfortunately the software is problematic to install and it is buggy -- very buggy. I've installed the software on four different Windows machines, following the directions precisely, and there were installation issues with every machine. With a couple machines I had to do some serious driver hacking to get the PVM software to start up. But even if you get the software to run, it is NOT intuitive and it is far too complex. All the software really needs to do is let you name/number a string, identify the gun/load, then record the shots and calculate ES, SD and mean. But the software is NOT intuitive -- There's a good chance you won't be able to figure out how to set up a string. There's also a significant chance the software will crash during your session.
So most folks leave the laptop at home and write down the numbers with a pencil and paper. That's an under-utilization of the PVM's capabilities, but most folks are forced to do things manually because of the crappy software.
If the PVM could somehow be mated to an Oehler base unit with simple buttons and a plain paper printer, we'd be in heaven. But that's not going to happen. And don't count on the software being improved any time soon. I've discussed this with the PVM's designer and he adamantly defends his program saying that all problems are caused by the Windows OS, not his programming. He's in DENIAL -- so he's not going to debug the software, nor create a simple USB interface so the PVM could work like 99% of modern peripherals.