You're here because the dime-a-dozen mice you can find at your local shop aren't what you're looking for, but also because you don't want to spend a quarter of a grand on some over-blown monstrosity that is just as likely to break as the aforementioned dozen. Outside of a weird corner-cutting spell some time ago, Logitech has consistently made great peripherals since Windows 3.1 and the G502 line is no different. So why not 5 stars?
1. The software is awful. G HUB is kind of in permanent beta status or something and doesn't work as well as you want it to. Fortunately, LGS works and is solid.
2. 3 of your 9 buttons are reserved, meaning when you drop into a game, you cannot bind them to "mouse n". The only way around this is to use G Hub (or Logitech Gaming Software) to bind these three buttons to actual keyboard keys and then bind them in the game. For the curious, these are the two buttons on the left-click button, and the aim button (which is actually nifty for temporarily dropping the dpi for when you're sniping).
3. Ergonomically, this mouse feels thin, but that's more on me. I bought this to replace a Sidewinder X5, which I would buy again if Microsoft was still making peripherals, but it is what it is. The X5 had a nice little fat butt and a small ledge for your ring and pinky fingers. I'd been using that mouse for over 12 years so switching to a thinner mouse feels weird and uncomfortable.
4. The mouse wheel is metal (or metallic) and while neat, is not a great material choice for something you need to grip with the pad of your finger unless you have grimy, sweaty hands all the time (no judgment here). OOB, the wheel is a bit stiff, but after a few hours of use, it loosens up (which is a bit scary - shouldn't it last longer?), but I still find I click the MMB more often than I want to (i.e. at all) when I am simply trying to scroll. This can be a problem in games where MMB is assigned to something. Speaking of the MMB, there's a button to unlock it so it spins like a fidget spinner. I can't imagine anyone would ever use this practically.
All said, it's just nitpicking. For the price point, you're buying an excellent, customizable mouse. The biggest downside are the reserved buttons, but then again, a lot of games don't recognize buttons assigned past Mouse 5. I cannot comment on the longevity of this thing, but I can't assume I'll get the same mileage out of it as I did the old X5. That thing was a workhorse.
UPDATE: From 4 to 2 stars. It's been just shy of a year with this mouse and the good news is the odd stiffness of the scroll wheel is no longer an issue. The bad news is the mouse will inevitably fast double-click on a single press. It doesn't happen all the time, but that exact issue is what I picked up the G502 to resolve on an older mouse. That old mouse lasted for 15 years (Sidewinder X5). This one has lasted 10 months. I highly suggest that you search any mouse model you're considering and add "double click" to it to see how many results you get. Apparently the G502 is notorious for this double-click issue and it's a gamble on whether or not it will happen.