Absolutly...especially if you are looking for high build quality, and extremely high light gathering capability at a 50mm focal length. It's a completely different lens than the 24-105 at 50mm when shooting wide open but if intending to use it at f4 or more it will look very similar IQ wise.
Absolutely, especially if the 50L would be your only fast lens. However, if you have both the 24-105 and the 50L, then it makes getting a 24-70 f/2.8 iffy, especially if you have a 16-35 and 70-200 as well.