shape/grip
AFAIK all mice in the MX510/MX518/G400/G400s line share the same shape. It's about the same size as my savu, but a bit wider and bulkier, which makes it more suitable for palm grip with three fingers on top (although fingertip or possibly even claw grip might work too). There's no pinkie rest, but a big, very pronounced bump on the right instead, which makes it much easier to pick up the mouse. This is actually a huge deal that can ruin the mouse for you, like it did in my case. First, G400 is fairly heavy for my tastes, well over 100 g. Second, the bump forces a weird angle between the pinkie and other fingers... maybe because my hands are too small or something. Perhaps if the mouse was more vertically oriented, it'd help; dunno.
Video:
Spoiler:
G400 is rubber-coated on the sides, but it's just coating, not rubber like on CM Storm Xornet/Spawn. The middle part is plastic with slightly granular matte finish, typical for Logitech. Coming from a new Roccat mouse, I wasn't impressed. OTOH, I like the relatively thin, not braided, soft&rubbery cable.
Build quality appears to be fine, except for buttons and the wheel. The main two buttons are normal, only the left one wiggles a bit—out of the box. The two thumb buttons are fine too; fairly tactile, well-placed, with a slightly rubbery finish. However, three small buttons on top are way too small, mushy and shallow. Moreover, the wheel, which is usually used more often than these extra buttons, is a disappointment... not because it lacks features of the four-direction hyperscroll wheel found on high-end Logitech mice. It just feels cheap; it's rubbery, but very narrow, loud when scrolling, and requires a deep press to click.
sensor
The sensor is Avago ADNS-3095 with 400/800/1800/3600 cpi steps. I didn't really test its performance though, because my pinkie hurt after only a few hours of use and then I didn't have enough time. It worked fine on my SS QcK/4HD.
software
AFAIK the mouse doesn't have an on-board memory for button remapping and stuff like that. Everything but sensitivity setting has to be done in software. I find this quite surprising, because cheaper G300 is better in this regard: it supports several on-board profiles. LGS is supported only on MS Windows and Apple OS X, I use neither, nor am I aware of any third party software other than universal system-wide remapping tools. Using LGS reportedly affects tracking and appears to be the only way to change e.g. polling rate, which defaults to 1 kHz.
conclusion
Try before you buy. The shape didn't work for me. YMMV. However, quality of the wheel and buttons on top, in addition to the lack of any on-board memory, was a huge disappointment for me, rendering the mouse barely usable.