Generally, Canon's L lenses live up to the hype however there are a couple you need to be warned about.
The Canon 24-70mm 2.8L :
Generally a wedding photographers all rounder these suffer from iffy quality control. There seems to be more problems with these than any other L Zoom out there.
Generally though any problems can be fixed by sending it to Canon for calibration , once fixed it's a superb lens, it's very heavy too, the 24-105mm F/4L is much lighter and has IS (depending on what you need).
The Canon 50mm 1.2L :
Generally this lens being 4 times the price of the 1.4 non L brother has been found to leave a bad taste in the mouth of many. Not so much because it's a bad lens but because the 1.4 version is so comparable. Unless you need the 1.2 aperture you might as well but the 1.4
Other lenses worth a mention (must have list) :
The 35mm 1.4L - Superbly sharp, fast, light and has fantastic colour resolution.
The 85mm 1.2L - Almost mythical lens , just perfect for portraits.
The 100mm 2.8L Macro IS - A portrait lens, a Macro and it has IS. It's also very light and versatile, recently considered the sharpest lens in the Canon line up.
The 135mm 2.0L - Every picture I've taken with this is perfect, almost always have a keeper with this lens, also the only lens I haven't needed to microadjust.
The 17-55mm 2.8 IS - Strictly for crop cameras only the 17-55mm lens isn't an L but has L quality optics and is one of the sharpest zooms I have ever used.
I hope this helps those deciding on lenses, if you need any advice feel free to email me.