I have been using a Logitech for some time, so this "review" is really a comparison to my old trusty MX Laser, which this mouse replaces.
First thing you notice is that the mouse if 30g lighter than the old MX. It may not sound much, but it feels a lot lighter. The other main difference is that the USB cable from the recharging unit to your PC is missing. All the transmitter components housed in the recharger are now in a tiny USB dongle, which just plugs in to the back of the PC and that's it. Setting the mouse up is a breeze, thanks to the Setpoint software, which comes with every and all Logitech mice.
Back and Forward keys are in the same place as the MX Laser, and extremely useful. The main wheel now has two modes of operation. Click or frictionless. Amazingly this all seems to be achieved mechanically, but the operation can be controlled by the software. Very clever, but I have to say that I am slightly dubious about it all. The frictionless mode very useful indeed, but I am not sure why anyone would want friction mode. Luckily it can all be configured to whatever you want.
The thing on the side that looks like another wheel, in fact isn't. It's just a switch that is a more complicated version of the switch on the MX Laser. Click it and you can use it to switch between applications. Not much use really, and I have to say that I configured mine for use with games! The other new switch on the mouse replaces the scroll up and down switch on the old MX Laser. This is a search switch. Once again, I am not sure how much use this will really be, but it doesn't matter, it's another switch that will no doubt be of use in games!!
My only one small complaint is that it's a bit noisy. the low friction wheel rattles when you move the mouse around. It also clunks a bit when the software switches it from friction to non-friction mode. I suppose there must be a tiny actuator in the mouse. Another minor difficulty is that I found the charger unit not as easy to use as the MX Laser. It always needed som moving around to get the mouse to charge. this is a relatively minor point though.
All in all, it's a great mouse. The frictionless, high speed scrolling just by itself is almost worth the high cost! But think carefully about wether you really need that function. If you don't, you might as well just save yourself a packet by going for something a little cheaper.