If you are not satisfied with the sharpness (it is hard to tell from
the web image how really sharp it is or isn't, or how it would look
printed), but if you would like it to look sharper, you can use an
unsharp mask, experimenting with settings for maximum sharpening
with least damage to the image. I find it useful to mask out the
sky (and maybe contrasty diagonal lines after the first stage of
sharpening - which may be a gentle one...) using a slight centered
feather on the mask edge. Since most wide angles are somewhat
soft in the corners, progressive repeated widely-feathered sharpening
can be done there to even up the sense of sharpness across the
frame bottom and/or other edges.
--
David Ruether
DRuether@twcny.rr.com
rpn1@cornell.edu
http://www.ferrario.com/ruether

What I feel is strange is the pixelization in the sky. The pics are
photographed at 100 Iso and these should be no such visible grain or noise.
Maybe it is the jpg compression in the camera? Try RAW-shooting and saving
to jpg at different grades of compression in Photoshop. Otherwise, I think
these pics show great sharpness and color saturation.
/per

It is not possible to comment on the sharpness of the lens as all you have
posted are resized images with absolutely no information in regard to the
settings and work flow used to process the images.

Firstly the pictures are acceptably sharp for nearly any purpose unless you
plan to print them very large.  After resizing down to whatever size is
needed, they will be very sharp if you do it right.  If you really want
super sharp looking pictures that can be maximally enlarged, there are a
some things you could do.

1. The original pictures are actually oversharpened which is trading real
detail for harsher edges.  Save sharpening for the last step after resizing
for final use.  This will preserve the maximum amount of detail and then
only sharpen the parts of the picture that need it.  Don't sharpen the out
of focus areas or the blue skies as that just adds visible noise and even
increases the file size.

2. For the three building pictures, you had bright light available, ISO 100
would have been fine on all three yet one is at ISO400 and another at 250.
This is needlessly increasing the noise which affects how sharp the image
appears.  You would have still had good shutter speeds even at 100 ISO.
This would have collected more light and lead to a smooth and sharp image
without as much grain.  By all means don't be afraid to crank up the ISO
when needed, even at 1600 the camera takes great pictures, but if you really
want maximum sharpness or enlargibility, then use 100 when you can.

3. The other thing is that you stopped down too far on the insurance company
building.  A 12mm lens has huge depth of field. F/20 is too small of an
aperture if you really were after maximum sharpness.  You could have got
away with f/11 or f/16.  The other two were at f/11 and f/13 but f/8 would
probably have been fine for those.  Too small of an aperture causes
diffraction, especially at short focal lengths where the aperture diameter
gets very small.  There's nothing wrong with stopping down really far, and
you should when needed, but it will affect the sharpness some.  You can get
away with stopping down less by choosing your focus distance more carefully
and putting the available depth of field where you want it.  For the two
smaller buildings, you have lots of sky in the picture.  If you had leveled
the camera a bit more and zoomed wider, then the plane of best focus could
be closer to vertical and a larger aperture could have been used as less
depth of field would be needed.

4. Finally your lens has a little bit of lateral chromatic aberration
visible.  This is where each colour channel has a slightly different
magnification and gives you the colour fringing on high contrast edges.
This is easy to correct in Photoshop especially if you shot the image RAW.
If it was a jpg, Photoshop can still fix it with the lens distortion filter
although not quite as well.  Correcting this will improve the image quality
and the apparent sharpness.

5. Take a few pictures of each, try different compositions to find the one
you like.  There is nothing wrong with keystoning as that is the perspective
when looking up, but sometimes it looks better to level the camera and can
allow a sharper image too due to reduced depth of field requirements.  The
one you took at 24mm could have been wider but with a level camera.  If that
puts too much street in front of the building or too much to the sides, then
crop the final image.  You have lots of megapixels to use.