I will make this review straight. Having used many gaming mice myself, there are few let downs that interferes the pros of this mouse.
Frankly speaking, the optical sensor of this mouse is quite good. Good in terms of accuracy in the shooting game. It's quite effective when you use rifles which has recoils such as AK47. This mouse is a palm grip type and so, it is quite easy to rapid click/fire during the game which is also critical in FPS game. The sensor is so good that I felt that the price of the mouse mostly goes to the sensor itself.
The size of the mouse is about 13cm in length and 6cm in width which is quite similar to that of Roccat Kone XTD but 1 cm shorter in width resulting an oval sort of shape just like a reptile egg in the dinosaur museum. Due to the high precision lead by the sensor, this shape does help. If there are too much width, it can be quite hard to control such sensor I'd say.
The weight adjustment is a great addition too. The sensor of this mouse is quite sensitive and so, the additional weight can give more control to the user in a way. Many people yell that this mouse is heavy. If you have used Razer and Cyborg mice such as Rat 5 - 9 (without the weight), and Kone Pure from Roccat, you may feel this mouse quite heavy to handle. But if you have used Roccat Kone XTD, then there is no problem as it weighs about the same i felt (with and without weight adjustment on both).
Lastly, the usb cable of this mouse is quite soft. The cable itself will adapt quickly and so, you wouldn't need to make it straight/stretch the cable after unboxing.
There are few down side of this mouse that may interfere with the good side listed above. Firstly, if you flip the mouse, you will quickly notice that there are four plastic black tape like seal for a grind protection. Normally, other brand namely Roccat and Razer won't do this. Instead, they completely seal them so that it won't peel off overtime. Here, I wouldn't worry about them peeling off but as a result of the wrong placement of the seal, this mouse chooses the surface. If you use Roccat Hiro 3D mouse surface for example, and move around the mouse, you will quickly notice the grind noise. A little delay caused by the resistance can be quite annoying when you want to aim the pointer properly during the game. Logitech gaming/non-gaming mice always gives this impression that they don't pursue for perfection, and that they are not made solely for the enthusiasts but also designed for ordinary consumers. This concept is still there with this mouse as well as the ones I used before from Logitech. This is my subjective opinion about logitech mouse products though.
The second cons goes to the combination of the wheel and the left/right click. It is certainly a nice addition that the wheel can be configured by a single click of button placed below the wheel. One click of the button gives a rigid feel of the wheel and another press gives a loose and smooth feel of the wheel. However, this rigid mode of the wheel can be an annoyance when combined with the mouse left/right clicks during the game or the normal daily operations on your PC. First of all, it feels very cheap. Secondly, the rigid feel of the wheel doesn't feel right, say while switching the weapons after few clicks of fire, the wheel accidentally moves due to the momentum of switching (scrolling) quickly. Thirdly, the clicks are loud. Louder than that of palm grip type mouse such as Kone XTD and in fact, louder than R.A.T 9 too.
The force you may put to click left/right buttons are just about right. At least, I feel this is quite comfortable. But if you switch from the soft clicks say from Razor, such as Black Mumba 2012 and DeathAdder, then you may feel these buttons need a little force to click.
There are people who complains about right clicks where they accidentaly press during the game but it never happened to me yet. This is probably due to the size of user's hand (mine is square palm with short fingers. About 17cm in length from the wrist to the tip of third finger) and the way you may place your hand on the mouse. Generally, if you place your hand as if you use the finger grip type of mouse, you may not have that accidental clicks.
Software and the drivers of this mouse works well and sensibly designed. I have never had any problems with updating firmwares when the software asked me unlike other unfortunate people who had to update the firmware manually through the installation directory. When you open the software, you will see this slide switch where you can choose where to save your profile. It is either the on-board memory chip built-in or to the PC. When you want to assign a windows function (assuming you are using Windows PC), you'd need to switch the profile save location to the PC by the way.
So there it is. Currently, I am using together with my Roccat Kone XTD and I can tell clearly, that this mouse wins when it comes to FPS games. I find it quite interesting that I use G502 for rifles that has recoils and for shotguns and heavy ones, I use the Roccat. The design of the mouse will remind you of the chinch bug or the beetles and like I said above, and you may need to consider buying one of the logitech gaming surfaces, but overall, I think this mouse is balanced and acceptable.
I bought this mouse as a replacement of my Roccat Tyon which I bought recently. Unlike the G502, the Tyon was a complete joke to me (I wrote the review of it too).
I am not entirely happy due to some of the half-hearted manufacturing especially the grind seal and the wheel which can be quite common in Logitech mice but the sensor itself is quite powerful.
When you pay for this price, again, I'd say you are paying for the sensor I thought.
I'd recommend this mouse to anyone who fed up with the laser sensor.