I have been using a Canon 70-200 f2.8 non-IS with and without a 1.4 TC for the last year for Soccer, basketball, Dance competitions Softball etc... I have been for the most part happy with the results. Now with my daughter moving up to larger soccer fields and I also will be covering the even larger soccer fields. I find my current lens limiting my shots or I have to crop heavily.
So I have been looking to possibly add a 300 f2.8 IS. I now have to convince my better half and myself that this would be a good "investment". I have been trying to research the value of the used lens market to see what this paticular lens is selling for. The reason being that if I pay $4k for a new lens and decide that it is no longer needed down the road what could I get back out of the lens. So my net cost is the difference which I need to apply to the value of the captured images during this time. My gut feel is that I could get 85-90% of the cost back assuming the lens is well maintaned. Couple years of use and 20,000 pictures later the $700 depreciation would be a good "investment".
You can check out some of my current images here.