As a Silver uMax user myself, I have to throw in my input.
This is one GREAT machine, whether you stick with the stock coil or add on the 12x10. The stock coil will, as Michigan Badger put it, get you at least 5 inches on coins. I've managed six inches, sometimes seven on pennies with just the stock coil and right ground conditions. I'm sure if there are any deeper, my 12x10 is not missing them. Coins and other good targets always give a good solid signal. I usually keep my disc down between iron and foil, and when I come across a good target I will up the disc with my thumb to see where it masks out. Generally this will give accurate results, but sometimes you gotta dig those "iffy" targets in between.
Since buying my Silver uMax in October, I have found a lot of great stuff including silver rings, silver coins, a gold-plated tie clip, Indian Head pennies, a very thin early 1900s era copper ring, a gold pendant and a silver pendant, buttons, tootsietoy cars, and lots of other goodies. I just don't see where you can go wrong with the Silver, particularly if you want to get a real machine for a downright ridiculous price. Not only this, but the compact size and light weight of the Tesoro line has always appealed to me being that I'm somewhat of a small guy (5'7", 150lbs). You can detect all day without wearing yourself out too much.
Ray