I was exactly in the same dilemma just 2 days ago. I ended up buying a used 70-200 f2.8L off ebay last night. I was going back and forth between this lens and the 135. I use 60D and 5D3, often missed the reach (especially in 5D), as I only had Canon's 24-70, 50, and Tokina's 11-16, and figured 135-200 ish reach would compliment my style. So I am planning to use this for a while and if I don't like it I know I will sell it off somewhere down the line and add cash to get the IS or 135.