I've been using this mouse for 8 months now. For years, I used a mouse with 12 side buttons for both gaming and work. Upgrading to 16 buttons is a long-awaited enhancement: for one-handed usage, the more buttons, the better, 12 were never enough.
Overall, I'm happy with the purchase. My main goal was to have more buttons, and in that regard, this mouse delivers. However, I hope the ergonomics will be improved in future revisions. Here are my suggestions:
-- The front buttons 5-8 are too far from the scroll wheel and have large gaps between them. As a result, I can only use the first 2, the others are out of reach. I had hoped to make better use of fingers other than my thumb, but the current design doesn't support that well - so those extra buttons unfortunately go unused. My previous mouse had only 5th and 6th front buttons, placed close together with no gap, and they were comfortable to use. I suggest moving the buttons closer to the scroll wheel, removing the gaps, and making the buttons smaller to make them accessible.
-- Side buttons are too long. First row position is okay (1-4), but the last (13-16) could be closer. My previous mouse had much shorter buttons and they were good, so this seems like a waste of space. I'd suggest either just shortening all side buttons to improve usability or shortening them and using the saved space to add a 17–20 button row.
-- The 8th front button is not customizable, which was an unpleasant surprise (though I can't reach it anyway, which is a separate issue). If I don’t need to switch profiles quickly, I prefer having the profile-switch button on the bottom of the mouse, like on my previous one. When using 2 profiles for the same application or game, I typically assigned the last front button as MODE+ in one profile and MODE− in the other.
Nice to have:
-- Split the 4th front button into 2 or even 3 buttons.
-- In general, more focus on front button design is needed. The thumb can easily be overworked, it's better to distribute actions across other fingers.
Ideally:
-- Add a tilt wheel.
-- Make the mouse vertical.
-- Make the mouse wireless, with support for a cable connection.
-- For gaming, adding a pointing stick on the side would be helpful.

Summary: The best programmable mouse at the moment - but with a few changes, it could be even better. It has several ergonomic flaws: the 7th and 8th front buttons are unreachable with the middle finger, the side buttons are overly long. The 8th front button isn’t customizable. With the ability to save and load profiles, 5 profiles are enough for me, and the software works well. The DPI configuration is good, 5000 is usually sufficient. I don't use weight tuning: the lighter the mouse is, the less fatigue my hand feels.

Very nice mouse, lots of cool buttons. Only problems are that the software is a bit buggy and that I completely removed all the weights from the base, even still its a bit too heavy for what I am used to, but that is okay.

Honestly one of the best MMO mice that I have owned in which I this would come as my first recommendation for anyone looking for one.

The build stability is solid, the aluminum base feels durable, and the long profile is naturally comfortable for people with longer fingers. The side buttons themselves feel responsive and have that slight click to them that adds to it.

Though, one gripe I do have about the mouse is the lack of disassembly guide or at least difficulty in getting one since mouse button one (left click) squeaks a bit when pressed towards the base and I'd wanna try to fix that. While it isn't noticeable when gaming, it can be a slight distraction otherwise.

It's ok, I like all the buttons. 13-16 do not input 13-16, they are separate keys like numpad / and such which can all be remapped. And one of the buttons on top is purely for profile switching, which I only need 1 profile, the software FORCES you to have 5 and making them all the same and saving everything is more frustrating than you'd think. Once it's all setup to your liking, it's a pretty decent mouse, though the 5-8 keys are very sensitive for me. I hope to see more options with a lot of buttons like this in the future.

I WAS A RIGHTY I LOST MY RIGHT ARM I WAS A GAMER IM STILL A GAMER A LEFT HAND GAMER IN ALL THE WORLD THERE IS NO BETTER PROGRAMABLE MOUSE I WORE OUT THREE SO FAR THE M990 IS REALLY THE ONLY MOUSE FOR ME