Recently I borked my M60 (one of the side buttons is loose and unresponsive) after about a good 6-7 months or so of use, so I decided to get a M90 which is now on sale (likely, stores are trying to clear M60/90 stock to replace them with M65/95 so getting a good deal is always awesome

).
Anyways, as I've now used both mice for some time now, some immediate differences I've noticed:
- the scroll wheel feels much better on the 60 than 90. the wheel has a "heavy" feel to it so you can feel each "notch" as you scroll up or down. the 90 scroll wheel feels much cheaper imo and less responsive.
- the 90 has a more rounded palm rest than the 60, and the surface of the mouse is more polished, so far it is more comfortable.
- side buttons of the M60 is much more 'clicky', produces a sharper click whereas the 90's side buttons are much duller in terms of the sound they make, but also harder to press. I use the side buttons frequently and I find that I had to put much more force on it, which is a minus for the 90.
- it's hard to access most of the M90's side buttons closer to the palm side. Corsair could've looked into the ergonomics of it more, those keys closest to the palm are nearly inaccessible in-game because you essentially have to alter your grip on the mouse to get to those keys. Not that I need them anyways (primarily a FPS/RTS player so I use the sniper key and the default keys for back/forward shortcuts in the browser), but just an observation.
- M90 feels much more lightweight, possibly due to the weights being a factor in the M60. I personally haven't noticed the difference before/after removing the weights when using the 60, but somehow the M90 feels much lighter in comparison.
- L/R clicks are comparable across the two mice.
Edited by supaflyballa - 6/12/13 at 1:11am