If like me you are a stickler for quality and would relish the prospect of trying out a lens for a few days before committing yourself to a purchase then take a look at
http://www.lenspimp.com/ - competitive hire rates for all those lens you would love to have in your kitbag but funds say otherwise. Great way of trying out a lens you may be after for relatively little outlay. Not bad when you may be considering at £1-2k lens. Please note that this service is currently only available for Canon/Nikon lenses.
Just got wind of this via a facebook add but thought it may be of interest to some folk on here
Dom