Whilst ADNS-9500 is the second most popular Avago sensor to date, after the 8200DPI sporting ADNS-9800, it doesn't suffer from the same acceleration problems like its bigger brother. Sure, there still is some slight positive acceleration, but it isn't something that actually has impact over one's gaming session, not for the average gamer, at least.
Thankfully, unlike the GX1000 with its hourglass figure and mildly uncomfortable edges, the Echelon can be used for hours without causing any kind of strain on your hand. It is designed more for the claw / finger-tip crowd, but even for a palm-grip user such as myself, its symmetrical shape is not a hassle to work with.
The sensor didn't have any kind of spasms or behave oddly whilst playing games that are taking in raw-input, like CS: GO or Battlefield 4 (option for raw mouse input are in the Settings Menu).
With only 8 programmable buttons, Echelon isn't really built for MMOs, MOBAs, or RTSes. I still played a bit of Starcraft 2: Heart of the Swarm, some Path of Exile, Banished, and lastly a bit of Wildstar. Overall, the mouse is not far from your average rodent, but I feel that the software might not be equipped to handle proper macro-building.
There's also a slight weight problem. Whilst the mouse seems tiny, it isn't really light. It has more than 100g without any weights added, and that might be a problem, especially to your claw-grip, low-sensitivty player.