Great machine and one of my all time favorites.
Treasure Talk feature is pretty cool, but I never really use it.
CX3 has a very deep non-motion true All Metal Mode. It's great because it still target IDs in non-motion All Metal Mode. The motion disc modes are all cool, too. The "Beach" mode has a salt elimination feature that's not really mentioned anywhere in the owner's manual, but Garrett's following machines had a "Salt Elimination" feature which could be selected for any motion disc mode.
The CX3 with PowerMaster circuitry get about 20% more depth over the original. Better target ID at depth as well. If yours has PowerMaster circuitry it will say in red letters on the side of the detector.
There's a wide variey of search coils available for the CX3 as well. You also have the option of using the Treasure Hound two box depth multiplier attachment which will find metal several feet in depth.
Here are some helpful links.
Field test report from Lost Treasure Magazine 1992 original CX3:
http://www.losttreasure.com/content/archives/garrett-electronics-grand-master-hunter-cx-iii
Field test report from Lost Treasure Magazine 1996 CX3 with PowerMaster:
http://www.losttreasure.com/content/archives/garrett-grand-master-hunter-cx-iii
Owners Manual:
http://www.garrett.com/hobbysite/hbby-manuals/Grand-Master-Hunter-CX-lll.pdf
Instructional video:
http://www.youtube.com/watch?v=OtQVIRE4T4I
By the way, $150 is a good deal.
Best of luck to ya!
Feel free to ask for advice at anytime. Happy Hunting.