Wicked. I like the design, neatly detailed and greebled. The hips look particularly good, nice job using the helmets.
I might comment on the open cockpit, but with this scale there's pretty much nothing you can do about it without impairing the look.
The shoulders look somewhat unfinished, though, with the holes and all that. How would it work if you stuck some grey minifig hands in the holes? Might give the shoulders a nifty spikey look.
You could also add some dark grey or silver highlights by replacing a few light grey parts; now, as the mech is mostly light grey but has a few differen't coloured points, like the other hip, it might look somewhat better if given a more solid colour scheme with highlights of other shades of grey. Might not, too..
And by the way, one more thing. I would also suggest replacing the grey robot hand in the gun with a black one or a black minifig hand. That'd make the rifle look more uniform and solid.