It is extremely comfortable to use and feels quite natural, especially over a full day of computer use. Going back to a normal mouse makes you realize how nice this one feels. So far, battery life is excellent. The top (thumb) button allows you to cycle through movement resolution scales - a good feature. There are a couple minor issues, though.
- The customization software is a bloatware download (that "supports" all mice/keyboards made by Logi) that doesn't really let you do very much; the claim of 4 customizable buttons appears to rely on the software, but the software doesn't offer customization of the buttons (?) - it's possible I'm too thick to figure out how it works. - If you want to use the mouse on two computers, back and forth, by moving from one computer's screen to another computer's screen, you do have to install the software on both computers. Too bad there isn't an onboard button that could substitute. - The accuracy is good - so good that the pointer often moves between when you depress and release the button; this usually makes the click "fail" (depending on the software) because you're off the icon/link before the release. I find this quite annoying - and the configuration software doesn't help you alter that. This stems from the mouse being so accurate and so easy to move. - I still knock my hand into the top of the mouse, occasionally, when moving from keyboard to mouse - decades of holding my hand at "the right height" to clear a standard mouse is quite ingrained. I don't know how many weeks it'll take to subconsciously life my hand high enough to clear the mouse. - I have quite large hands and it still feels a tiny bit too big; my wife's smaller (normal) hands don't really "fit" - she wouldn't use it. - The thumb-side double buttons are positioned directly through the mouse from the left-click finger-side (normal side) buttons; this makes it nearly impossible to thumb press either of the thumb side buttons without also depressing the finger-side left click. You can do it by "training" your hand to hold the mouse with your 3rd and pinkie finger only, but this feels awkward. On the plus side, the thumb-side buttons are well above the normal thumb position so you don't ever accidentally click them. - The cable (recharging or corded use) is a non-standard USB cable. Why? Only the packaged cable will plug into/recharge the mouse. Major drag. There is an on-board charging light which is nice. On the positive side, the cable does "click in" to the mouse quite firmly.
And yes, you will get lots of strange looks and a few questions about your weird-looking new device.