But first: Installation under Windows 8 was as usual a no brainer. The M555b I never had any problems, only once and it was up to the drivers of the used Bluetooth Modules PC side. Always use the original driver from Windows When in doubt, or if necessary, for a few euros a Bluetooth dongle bring the exotic no drivers are required.
Now about the differences: Both mice see at first glance look the same, differ primarily on closer inspection by the imprint on top and bottom. When you turn the mice then sees the hole for the laser diode varies. The closure of the battery compartment of M555b acts clearly valent thanks a spring mechanism, but it shows of course only when changing the batteries. The M557 is 8 grams lighter and weighs 107 grams with batteries with me. Quite difficult for a mouse, located some will think, I personally like it when the mouse has weight.
Only real difference: When M557 mouse wheel is again a third button, the old model could by pressing the mouse wheel simply adjust how loose, this can be turned. I did, in fact, had ever held her for absolutely unnecessary.
Otherwise, offer both mice next to the classic left and right mouse button additionally a button below the scroll wheel (which I to switch between apps use), and the ability to tap the scroll wheel left or right (I use for pre-trip).
Conclusion: Of course I have not tested the new M557 long and detailed, but the first impression I notice no difference to M555b with which I am very satisfied for years.