There's precious little out there on this mouse yet, so I figured I'd add to the flood of information.
Coming from Logitech optical, to MX310, to MX500, to MX510, to MX518, the safest move would be the G400, if I want LGS (I do). But the G300 was just too odd to not try out.
It feels like a much smaller mouse than it is, thanks to the flat shape and inset grips. The extra buttons are much easier to reach and use than it looks they would be, too. The scroll wheel has a great feel to it, as well. The sensor position is really odd, but I didn't notice any oddities in using it, with a hybrid claw/palm grip (rarely palm when gaming).
You only get to choose DPIs among the actual sensor values, from what I can see, which given how many there are, is cool. I went with 750/1750/2250/2500, since I'm used to 800/1800 (MX518), and must have 4 settings.
Scripting is broken. If I downgrade LGS, scriptinig works fine. If I boot up without the G300 plugged in, it works fine. If I plug the G300 in, it disappears (even from the G13--something's borked w/ the G300 support!). It's supposed to work with the G300, now, but instead, it's just gone from the software.
Now, both of those can be fixed. Not only that, but when the 2nd gets fixed (it will), the first should be able to be worked around.
The real killer is tracking. My pad is an Allsop XL raindrop, which my MX518 can get pixel-perfect tracking on at 1800 DPI, if moving too slowly for prediction to kick in, and my G9x can get pixel-perfect tracking at up to 2800 DPI on it (just shy of 5700/2--coincidence?). Above 1250 DPI, the G300 acted like it had medium/strong prediction, while it was perfect up to 1250. Very noticeably at 1750, and worse if I increased it more. I use 1800 outside of games, so that's a deal-breaker. I tried it directly on the tabletop, and on an old Goliathus Control, and got similar tracking. On the bright side, it acted like good optical mice tend to, and never missed a single reflexive aiming movement.
Since it officially doesn't use prediction, it's faulty, IMO, as it does not function correctly at over half of its rated DPI. IMO, if they used a sensor like the MX518's (or maybe G400v2, which I'll be getting after I return it), or G9x/G500/etc., it would be a solid small mouse. As it is, it may be a good mouse if you only ever use low DPI.
P.S. the bug got to me, and I got a G400 B&M (Logitech on sticker, no PID). Adjustable number of DPI settings (so probably coming to G300, edited that out), no angle snapping, and everything seems to work perfectly in the software, so far, unlike the G300. It's like 2003 all over again, but this time with smooth tracking and Lua