My default is usually in favor of L glass. The exceptions have been for a Sigma 50mm f/1.4 (pre-Art) and a Sigma 12-24 MkI. A Sigma 30mm f/1.4 for APS-C was a completely forgettable, almost unusable abomination.
The 50 f/1.4 was occasionally heroic wide open, but the hit rate for accurate AF was dreadful. I couldn't trust it and never once used it for commissioned work. AF inconsistency with a 50mm lens shooting at wide apertures is ruthlessly revealed. Pity, it was just fantastic when it did nail focus. Sold after a few months.
The 12-24 is a lens I use a few times a year when the 16-35 doesn't quite make it and the situation won't readily allow for stitching. It's not a fabulous lens, but I have likely scored a good copy. Wide open it's complete mush, but at its sweet spot of f/11 it delivers files acceptable for most usage requirements. I believe the updated 12-24 is a great improvement. Any AF inconsistency with a UWA stopped down to f/11 is unlikely to reveal itself.
Sigma will see radically increased sales once they fully consign their AF issues to history and the perception of AF unreliability has faded.
That's not to say every L lens I've ever bought has been a keeper. The old 16-35's were mostly terrible and the MkI 24-70 f/2.8's were very inconsistent from copy to copy. There are good ones out there, but tend to be the exception. A 50mm f/1.2L was a short-lived relationship. This lens does have its fans, but it never completely cut it for me. The EF 50mm f/1.4 while pleasingly compact, was never the best lens in the bag.
This year I've rationalised my lenses, selling off surplus, under-performing or obsolete glass. What remains is 16-35 f/4is, 24-70 f/2.8II, 70-200 f/2.8isII, 24 f/1.4II, 100L macro, 300 f/2.8is and the 12-24 Sigma. The two least used lenses are the Sigma 12-24 & the 24 f/1.4II. Hmmm...may as well sell the 24 f/1.4II too. Unused this year.
-pw