Hnngh... I wish I could draw mechs. I really like the second one in particular, the rounded armour makes me think of what Shirow used to draw, back in the day.
For your people though, I think anything I say is probably stylistic nitpicking, so feel free to take it with a pinch of salt.
The first two women seem to have very rounded heads. I know only too well that avoiding man-jaw can be a difficult job sometimes, but I think you could easily get away with a little more chin there. To my mind, it looks more natural and it can be a big help when trying to differentiate one face from another.
The photoshop shadows aren't quite working for me either. If you're adding a solid shadow-layer on top of existing colours, I don't think it's worth blurring it as heavily as that. If you're putting in spot blacks, I think they tend to work much better with a really strong, sharp outline around them to really give the image definition and impact.
I do like the first girl though, she looks like a gentler version of Revy from Black Lagoon.