Rented Canon 300 mm F4L I/S lens from Penn Camera (Tyson's), $35/day, for try-out shots at Trade Zone training races. (Images posted here.) I usually shoot 70-200 F4L zoom. I like the zoom because I can do more variety -- e.g., close-up people shots, candids, mild panorama. That said, the 300 was something else. Amazing sharpness, gorgeous color (despite drizzle).
Grey, misty, about 45 degrees. Shot at ISO 800, action shots 1/1250 or 1/1000, candids less (1/400 +/-). Lens wasn't too heavy, handheld just fine. Good reach into race action. Nice bokeh (softness behind shallow depth of field). Left about halfway through the 'A' (fastest) race; rain become more steady, didn't want to gum the lens. The rental lens improves outcomes, but at $1240, I'm not ready for kitchen point deficit.
Follow-up note: Used 300 mm with IS mode 2 (panning). Will try it next with IS turned off to improve focusing speed.