I just purchased the 50mm 1.2L USM, and after stumbling a bit at first with it, am really starting to love it. This gives me weather-sealing, which is relevant at work, where I must shoot in all weather, and in the backyard, where there is a pool, and at my nephew's swim meets. I will not normally shoot wide-open, but the option is certainly there, when I want the shallow depth of field, or want to freeze action in low light.
Now, with two 7D bodies, and 100mm and 50mm weather-sealed L lenses, I will rarely have to switch lenses.
Canon 7D2/7D/5D/40D/1D2N/M3; Nikon F6/D700/FM3A/1Dx/Coolpix A; Canon 40mm 2.8 STM, 135L, 50L, 35L, 50mm 1.8 I, 100mm 2.8L Macro, 10-22mm EF-S, 400mm 5.6L, 16-35mm 2.8L II; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S, 45mm 2.8 AI-P, 14-24mm 2.8G, 24-70mm 2.8G, Micro-Nikkor 60/2.8G; Voigtlander 90mm f/3.5 SL II, Zeiss ZF.2 2/135 APO