There are no surprises here. The G400's shell is the same exact shape as the mx and g5 mice we've been using for years, which is definitely not a bad thing. It's a right handed ergonomic shape and designed for palm or hybrid palm/claw grips. I'm a huge fan of the design and especially love the little lip that is on the right side of the mouse. It makes lifting the mouse up take almost no effort, and because of this, I have a hard time even using other mice because I feel like I can never get a good grip on them when lifting off.
Thankfully, Logitech ditched the glossy plastic in favor of regular, painted plastic for the top/palm part of the shell. No longer will you have to wipe gunk off your mouse after every gaming session. They said a smooth, matte texture was used for the top. I guess it feels slightly different from regular plastic, but there's really nothing special about it. The sides of the mouse are coated with a nice smooth rubberized coating. They went with a simple dark grey color scheme and it looks quite nice and minimalistic.
The cable is non-braided, thinner, and more flexible than the mx518.
I'm guessing the G400 uses the same sensor as the mx518 except the dpi now maxes out at 3600 and the polling rate at 1000hz. Out of the box, the dpi levels are preset to 400/800/1800/3600 and the polling rate to 1000hz (edit) 500hz (You can set it to 1000 via software). Testing showed that there were no acceleration issues with any of the levels I tried. I was using a Steelseries Qck for my tests.
Like the mx518, the G400 has prediction. Nothing much to be said here. Some don't mind it, others dislike it /shrug. Personally, I don't mind it and would rather have prediction than, say, acceleration problems.
Lift off distance is fine and low. I'm not a lod snob, so unless it's pretty high, I'm not going to notice anything wrong. (edit) The mouse stops tracking when sitting on a stack of three dvds.
After installing Logitech's software, I found that I could actually set the dpi as low as 200. I tested it, and there were no acceleration issues. However, when testing post-software prediction, I noticed there was extremely noticeable jitter when drawing lines as seen in the picture below. This occurred at any polling rate and dpi setting. As soon as I exited the software, the jitter would be gone. I'm not sure if they were trying to mess with disabling prediction or something, but there is something wrong here. I'm going to make a post about it on their support forums and see if they have anything to say.
I have no way to accurately measure the weight of the G400, but it's supposed to weigh 105g without the cable, 133g with it. It seems around the same weight as the mx518, Deathadder, and G5, maybe even slightly lighter. Honestly, they all feel so similar, it's extremely hard to tell without actually weighing them. I'm sure there will be some accurate info out there once someone with a scale gets one.
The G400 I received felt pretty solid. There's no play in any of the buttons, but my lmb does have a slight wobble to it. It doesn't affect the act of clicking it at all and once your finger is resting naturally on the button, it's a non-issue. This is most likely one of those things that is going to vary from mouse to mouse. The rmb/lmb and side buttons feel solid, have great feedback, and don't take much pressure to depress.
The scroll wheel is, unfortunately, similar to the mx518. It feels pretty cheap, and the wheel I got actually feels worse than the mx518. It's hard to explain, but the notches have a rough feeling to them when scrolling. It just feels cheap and lazy. The mx518 had a problem where if you were scrolling fast, the wheel would sometimes pick up scrolls in the opposite direction. I've been testing this and it seems to still happen, but less often. The middle mouse button takes a decent amount of force to depress.
The rubber coating on the sides feels fine and seems to be the same stuff used on the mx518's sides.
The G400 doesn't use Setpoint and, instead, uses "Logitech Gaming Software". I actually like the way it's set up. The application is clean and easy to use. You can create macros now, which is nice if you're into that. As mentioned above, there seems to be a pretty big issue with jittering while the software is running. Until that's fixed, you're pretty much forced to run without it, meaning a forced 500hz polling and 400/800/1800/3600 dpi steps. Hopefully that issue is fixed.
edit: Realized that it runs at 1000hz out of the box, not 500.