I recently rented a Canon EF 70-200mm F2.8 L IS USM II and a Canon EF 2X Extender and spent a week taking photographs of anything that flew. My first impression was how heavy the lens and extender were but by the third day I was used to it and it didn't seem so bad. The auto focus was slow compared to my usual telephoto and sometimes would not focus at all before the shot dissipated.
These were my only complaints and I was very happy with the results considering almost all of my shooting for the week was hand held. This Black Phoebe shows the detail achieved at 400mm which is the maximum for this set up. Frankly I wish I had more time with this package but at $172 for the week it wouldn't take too many weeks to put a good down payment on a new one. It is nice to know that you can rent lenses when you want to do a special project because you would have to spend a fortune to own all that I want.
My next rental will probably be a super wide angle so that I can do landscapes for a week perhaps at the Grand Canyon which is only 350 miles from here..