For almost 5 years, my standard lens for aviation shooting has been my Canon 70-200mm F/4 L, which has been a trusty lens, very sharp, and a good length on the APS-C sensor on my 30D (112mm x 320mm equiv). However, switching to the 5D mark II has reduced the range. For a while now I've been contemplating the Canon 100-400mm F/4.5-5.6 IS L lens, which is very popular among planespotters for its range and sharpness.
A friend of mine had like'd a lens rental outfit in California called BorrowLenses.com, so I checked out their site and they had listed a used 100-400mm for $1350, a decent price, but by the time I secured the funds to purchase it it was gone.
Then a fellow in Renton posted one on seattle.craigslist.org for $1350, but I missed the opportunity, however, he was nice enough to forward me email address on to a friend of his who was also selling his, slightly older 100-400mm for $1300. Done and done.
My first chance to really use the lens came the weekend following, as there was a B-17G and P-40E Warhawk visiting Boeing Field. So I spent a few hours catching them and the lens worked out as I had hoped. Great sharpness and great range.
The IS took a little getting used to on the panning, I eventially set it to the single-axis (mode 2) panning and that seemed to work best for me.