I hired a 1.4x teleconverter for the day, to give me a longer reach, and definitely found it useful. At only $15 per day, the cost of hiring the teleconverter was definitely worthwhile!
The planes are great to watch, hurtling at over 300 km/h just a few metres above the river.
Here's a couple of my photos from Saturday.

Canon EOS 50D, 70-200mm f/4L @280mm (1.4x teleconverter), 1/200 sec, f/13, ISO100

Canon EOS 50D, 70-200mm f/4L @280mm (1.4x teleconverter), 1/200 sec, f/8, ISO100
So I'm gathering that you rented some sort of special lens for $15 for the day...my question is, how expensive are they to buy? Do you think you wouldn't use it enough to make it worth owning your own rather than renting one?