A question often asked here is "should i buy x lens?".
Often people compare offerings by Tamron, Sigma et al with original lenses by (usually) Canon and Nikon. Price is generally a significant factor.
Lately I have bought several L lenses and have looked at eBay, imports ( I used HDEW cameras very happily) and also my local dealer (who I bought my 70-200L IS II f2.8 from).
It is hard to compare Canon's pro lenses with offerings from other manufacturers, as ruggedness and weather sealing come into the equation as well as performance and price.
What is very apparent though, is that pro glass holds its price incredibly well if you buy Canon and Nikon. In many cases used prices on eBay actually exceed the price of new glass available from importers (full Canon guarantee). Depreciation is minimal. The lens can be used for a year and sold on (if you wish), with a price differential equivalent to renting the lens for a couple of weeks. There always seems to be high demand for L glass or equivalent.
The same holds true for used equipment in dealers.
Clearly you have to fund it in some way it as the initial outlay is quite high, but the economics of ownership make sense to me (I never borrow money).
This does not hold true for the Tamrons and Sigmas though. If eBay pricing is used as a guide, depreciation is heavy (circa 40% in a year). This usually outweighs the cost of ownership of the OEM lenses.
I don't think I have seen this crop up in lens purchase advice.
Adrian