The L lenses (luxury I believe is what that stands for - read something about that years ago) are great quality. The optics are superb, but not are all immaculate.
For example, the 16-35L (not the MKII) had softness problems on the corners. The MKII is supposedly better. The 17-35L was soft on the corners as well, as is the 17-40L on a FF sensor (or film). On an APC body, I noticed no such problems with the 16/35 or 17/40.
If you shoot with live view and manual focus, there are tons of superior lenses to what Canon makes. Of course you lose AF, unless you have someone convert one. I saw a Contax 17-35 go for $3500 on eBay, and it was converted.
I am going to be buying a Canon FL 50mm 1,2 and have
Jim Buchanan convert it to EOS mount. It is a superb lens (many say equal to the 50 1.2L, but that is all subjective), and worth the conversion!
As with lenses, (perhaps minus the Zeiss, Contax, Leica, Voightlander..) there are variations in copies. I cannot tell you how many times I have read about people sending in a lens to Canon for calibration (and then Canon wanting them to send in the body as well). Seemed to happen a bit with the 85L (back focused), the 50L, 24-70..
You can't go wrong with primes! If you can live with the redundancy of switching lenses, that is the best bet. Constant aperture (and quite a bit faster), and sharper. Even the non L primes are very good, but not all of them. The 50 1.8 is proof that a cheap prime can be sharp.
Just remember that there ARE excellent EOS mount alternatives. My Sigma 10-20 was as sharp as my 10-22mm, and cost a bit less. The Tamron 28-75 (or whatnot) is a very good lens as well, as are the Sigma primes. Just do your research. FredMiranda's forums have 10 years worth of info, just search. Look on the nature forums, and you will see many that shoot with the Bigma (Sigma 50-500), and the results are amazing. Same goes with the Sigma 70-200 EX. I know that if I had $1300, I would by a Zeiss over an L. If you need AF, perhaps this may not be the best idea (as a conversion is not the cheapest and there is a wait, from what i gather)