I got the M18 backpack switchtank sprayer when it came out, and it was absolute rubbish, but it's biggest issues all came down to the pickup at the bottom of a flat bottomed tank, leading to the pump sucking air with a gallon still in the tank and a strainer that always clogged. Also, their system liked to leak out the bottom.
tl:dr; the M18 backpack sprayer is a terrible product - stay away.
The M12 sprayer shouldn't have any of these issues, and I would fully expect it to be a great system.
What I like about the M18 sprayer (that should apply to the M12):
The switch/valve wand that disables the pump when you release the valve handle is a really nice touch. It's also a good wand, and I really have to emphasize that, because most plastic handled wands eventually end up leaking product on your hands, and this wand is built nearly as nicely as a B&G all brass wand.
It takes standard Teejet nozzles, though you'll want to find an angled adapter. If you know what nozzles to order, you can get the exact spray pattern ideal for your application. Foamjet nozzles too.
Very good battery life. FWIW, spraying doesn't really use much battery.
Much better pressure (more power available, and more importantly more consistent pressure) than hand pumped sprayers.
That being said, there are lots of companies putting out quality tank-topper spraying systems today. Ryobi and Harbor Freight's are probably just as good as the M12 one and cost quite a bit less. After my M18 backpack debacle, I ended up DIYing my own using a 12V buck converter, and M18 battery adapter, a 12V RV faucet pump (it has a pressure switch shutoff, but I also added a pendant electric shutoff so I can more easily stop it when it runs dry), and I made my own wand using pressure washer extension tubes, a Milton blow gun with a 1/4 NPT output, Teejet adapters, brass air QDs and air hose. The setup cost me nearly $200, but it's also much better than anything I could buy.
