... I'm surprised you were happy with the results from the 75-300, my wife has it as well and is probably the worst performing lens we have between the two of us, ...
There is more to photography than pixel peeping. Neither of us ever shot pictures of brick walls, just action sports. I've had a few landscape half pages in magazines with this lens. Whats not to like?
Last time I used the 75-300mm was on a 40D at a Western Surfing Association contest in January 2008. I shot off the pier for about an hour, and got several shots that made good 8x10 and 12x18 prints. Normally I use either a 300mm f2.8 or 400mm f2.8, mounted on a Wimberley II gimbal head, for my long lens work.
Remember good photos have nothing to do with ultimate sharpness, barrel or pincushion. It's all about the response that the photo evokes.
BTW I bought the Wimberley gimbal head in 2006, and it was one of the best investments I've made in photo gear. Check-it-out http://www.tripodhead.com/products/wimberley-main.cfm