I'm going to have to say don't like the buttons. The dither you have looks grainy and your circles look like they're made with the circle tool; you could use to touch up their curves. Some inner aa on the buttons would be nice, and it's possible even with your current palette.
The big numbers aren't bad but look like you could be using more dither patterns. Currently you go:
50%
0%
50%
50%
0%
50%
50%
50%
0%
50%
50%
0%
50%
You should try something more like 0-25-50-25-50, but tweaked to fit the length you need.
The ship pointing right has good shading in most areas, but the front cylindrical part and the bottom fin aren't really shaded to form right. Extend the cylinder's highlights and shadows towards its center and consider taking the outlines out of the inner part of the sprite, especially the ones of the pyramids. Outlines generally waste space you could've used for conveyance of form or detail, and take away from realism. If you do decide to use them, they work best on pieces a bit larger than this where the outlines don't end up taking up so many of the available pixels.
The ships pointing left are very strange. The lightsource is opposite that of the other ship, and I'm having trouble reading any 3D objects from them. What are the little grey bits? How would the ship look from the front? Here, too, I suggest eliminating outlines to gain space for details and form, both absent in the smaller ships.
