The two tops ones are default bound to sensitivity control but, of course you can remap this to whatever you want. I've currently found no need for the G buttons though. I would have been happy with just two buttons and a clickable wheel.
I like it because it performs well and sips battery at the same time and it gives you control over how quickly to poll. It's a lot of mouse and it works well IMHO.
I don't use them either but that's because i am left handed haha.
Their was 2 reasons from what i can see that i picked the G700s over the G602 which would be the dual scroll wheel which i must admit totally love, battery has not ran out on mine yet and plug it in here and there when i please even when it says 10% i still get about 2 hours easy out of it.
G700s dual scroll is a plus, if you don't like that sheesh WTF. only time i can think of when not liking it would be forgetting it's not on the mode you need. It's a great feature in Elite Dangerous too for the system map
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G700s battery time i get about 10 hours + with max power rate and polling rate, how ever this may not be long to some people you can use other rechargeable AA battery's which i have 2 on standby but never use and i forgot to plug it in pretty often lol, so to me all a plus.
G700's sensor is at the front so once used to the mouse you can just left the front of the mouse so no need to pick the whole mouse up on the pad.
G700's works perfectly on my glass mat.
G700's allows me to pick 3000 dpi which i use mainly in windows, while gaming i like around 700-2000 while gaming, i use 2 of the top buttons for that and the other for teamspeak how ever the buttons could be better.
G700's wired mode is complete crap due to cable supplied plus the added weight.
G700's when i got it was only $50 which i believe is a steal and not to bad of a deal at $65.
Anyways it be the dual scroll wheel and the fixed battery that's in the G602 that would of turned me away from it.
But still i do miss my Sensei due to the scroll wheel being better as time to time swapping weapons while trying to click the middle mouse button is deadly
, how ever you do get more used to it over time so becomes less of a issue.
Some thing that a right handed person would probably hate would be the squidgy feeling of the G buttons, but seem to press well on mine at least.
EDIT:
Should have my Sensei wireless some time wednesday i dunno what i am going to do as i loved the wired version but it failed in 6-8 month. Don't know if i should keep it or sell it and buy a different mouse, shi i don't know if i am going to even allow my self open the box and just sell it lol.
Forgot to add that the G700s uses a single battery unlike the 602 which has gotto add some weight.