Thinking about buy one of these and wanted t know.... 1910, 1915, 1920, 2210, 3850, 4150, 5450, rz1710, rz1717, rx3100, rx3400, hx4700, hx4705, h6310, h6340, hx2110, hx2415, hx2750, hw6510, hw6515. Whats it all about? what's the difference between rz, rx,hx,h, hw and ones with no letters?? The one I've found that I might buy is called 3870, I'm just trying to figure out what it all means and how good/new this one is. Any info would be appreciated. Thanks all G /edit....also those of you who have them, what do you personally do with them?