I'm not sure what you already know about the round, but I'll cover the basics. It's a 10mm case necked down to accept a 9mm projectile. It's the exact same concept as the .357sig, except it uses the 10mm case instead of the shorter .40s&w case (which had the 10mm as it's parent cartridge anyway). In short, it's one of the most powerful 9mm cartridges there is. Your factory ammo selection is very limited... pretty much DT, Underwood, and perhaps BB. Finding any of these for less than $1.00 per round will be a challenge. Reloading for it isn't cheap either. Dillon is the only source for dies that I know of, and be prepared to shell out $150.00+ for them. Also load data for it is scarce, so reloading for it is not really for the novice. I've read of some using .357sig max loads as starting loads for the 9x25, and they work up from there. Published "Do Not Exceeds" are scarce to find, though, and it's often up to the intelligent and careful reloader to know when enough is enough. I believe Lyman has a few, but covering only a few powders and projectiles (5 of each at best)
That being said, the round launches 9mm projectiles at truly reckless speeds. It appears to work best with the lighter weight projectiles, from what I have read from other's experience. If you are looking to launch 147 gn or heavier projectiles, the caliber has little advantage over 10mm. If launching 125 gn projectiles to 1500+ fps is your thing, then 9x25 Dillon is where it is at.