I have what I believe are two M3 adjustables. The descriptions on the usually cited Schick page do not always quite match what I see.
I used one for 10 straight shaves earlier this month. Worked great, and the adjustment does make some difference in how it shaves. I ended up just putting it on 7, the next highest setting, and leaving it there. For me, I don't know if there is really an advantage over my G or I types, which are about right to begin with. Someone else, who wanted a milder razor, might like the option of lower settings. The trouble is, the adjustables seem to sell for a lot more on eBay at this time. No, I'm not looking to get rid of my spare right now.
“The world wishes to be deceived, therefore let it be deceived.” - - Petronius