I have the G700, G600 and G500 so here's a quick rundown of each for those on the fence.
G700 is the "spiritual successor" to Logitech's wireless mice suitable for gamers, like the MX700, MX1000, G7. The shape is nice, kind of a contoured classic gamer mouse with a nice thumb landing but not as fat in the hand as the G9/x. Texture is a bit rough on the sides to help mask dirt/grime build-up. One thing to note is that this mouse has 2 well-documented problems. First is that it suffers from some pretty significant loss in accuracy, polling rate and consistency when used in wireless mode. I don't know if they ever fixed this via driver or firmware, but last I checked they had not. As a result I game with it always connected over USB and use a mouse cord bungee because the stock cord is super thick/heavy. Secondly, there is a pretty good chance the left mouse button will eventually start flaking out on you if you game heavily. Basically, it starts clicking erratically to the tune of no click/double-clicks when you don't want it to. Very frustrating. I had it happen to me, but if it does happen to you, Logitech warranty support is top-notch. The G-keys at the index finger are great, I use them to bind to alt-shift-ctrl to bring up more options in MMOs for keys 1-6. The side keys are too thin and awkward to be great and aren't actual buttons, they press down on pads so they don't feel very responsive. The feet glide well but tend to pick up gunk with 4 small pads spread across the bottom. Overall this feels the most quality of all the mice on sale by a good bit.
G600 - I recently bought this for my gf because she wanted something like this, or the Naga for MMOs like GW2. It's a solid mouse from what I tested. Feels good in the hand, a bit fuller similar to the G9x, but the quality of the top buttons/housing definitely felt looser. The side button array felt really good with a much better responsiveness than the G700. This mouse glides very smoothly with 2 very large pads similar to the G9x. The mouse body is smoother overall, you don't get the rough texture like the G700. Good weight to it without having to mess with weights. If you are buying a mouse strictly for wired gaming, I think this is probably the better mouse overall. I haven't really used it much, so I can't give too much insight on how it actually performs.
G500 - No frills gaming mouse. I think it's the spiritual successor to the super popular G5, the G400 would be the closest to the MX518 for the purists who swear by that mouse. Again, you get the textured surface on the sides to mask dirt. Not as much flexibility with the side and index finger buttons in terms of key mapping, and they are also much stiffer and less responsive imo. This mouse feels the cheapest but not cheap, and you may want to add some of the included weights to give it some heft. For $15 more I would probably go with the G700 or G600 over this.