Thanks Andrew - great overview.
Apart from the cost, I find Canon L glass to be large and heavy. The 24-105mm f4 IS is an exception because this weather sealed kit lens is very versatile. Depth of field is approximately as shallow as an f2 lens on a GH2 body. In use it's noticably smaller and lighter than the 24-70mm f2.8 L lens. The best feature is very quiet image stabilization which is usable handheld through the entire zoom range.
For under $500 my favourite wide prime is the very small and light Canon EF 28mm 1.8.
Obviously it has less distortion than a 24mm lens and has full time manual focus meaning I can use auto focus check and then very quickly override with the wonderfully smooth manual focus ring.
All photographic lenses are designed to resolve far more than 2K. So even if I think I may need L glass, my results show it's often unnecessary. My (non L) EF primes are mostly sharper than my L glass at optimum apertures. Many of these primes display very acceptable behaviour (bokeh, flare, distortion etc, etc) for the money.
For under $400 I use the EF 50mm 1.4 and the EF 85mm 1.8. All are fast, light and very sharp with very good build quality. Smooth manual focus on the 28 and 85 is superb.