I now have a shiney new 50mm 1.8 lens which I'm itching to try out. things this will help with: higher-end photos where clarity throughout the frame is needed (this lens generates less edge-distortion than my zoom lens) and faster shots in low-light situations (it can gather roughly double the light gathered by the zoom lens).
relevant appointments can now be made.
:addendum: I've done a lens comparison using various objects with fine detail and a range of colours. The 50mm is FAR superior to the 28-135 in terms of clarity and line distortion across the picture. The colour distortion of the 28-135 is not readilly noticable to my eye on this monitor. That edge-blur that the 28-135 has, though, is painful now that I'm seeing what a nice clear lens like the 50 can do! :) The fixed focal length is a bit annoying, but it's a reasonable trade-off.