Have been using 800 DPI for as long as I can remember. Dead-middle on Windows sensitivity, as I've read that not having it right in the middle can make your mouse really inconsistent.
I have many friends that use insanely high DPIs because they think it's better, completely ignoring that the huge majority of pro FPS players use 400 or 800

It's not that I want to be a pro or that I think I am one, but I've gotten comfortable using those settings and I definitely think low sens + big mousepad has improved my ability to play FPS games.
I'd probably use a bit higher if I played MOBA or RTS games, though. I'd also consider using a higher DPI when browsing, but I use a ZOWIE FK1+, and the DPI can't really be adjusted except when the mouse is just turning on, so I just roll with 800.
At work, I use a Logitech MX Master, but I've never installed the software for it, so whatever the default DPI on it is, I use that.