Since I'm a nimnal photographer, I don't think I can justify purchasing or renting. But my wife was an excellent photographer, and she spent a lot of time pondering just which lenses to purchase. I suppose if you really want to test out a particular lens and be sure before you buy, renting would be a good option. On the other hand, if the lens is expensive, maybe a camera shop would have a try-out available? Or maybe the "rental/try-out" fee would then get applied toward the purchase price? That would protect them from having someone try out their lens and go buy it somewhere else, and allow you to get the benefit of the money you already shelled out. I'm just thinking out loud here.

I've used http://lensrentals.com as a "try before you buy" service many times. In some cases, spending a little money saved me a lot of money. Their customer service is excellent. I had a problem with a lens only once, I reported it, and they sent a replacement. (always take the extra insurance... you never know)

Why not buy it... mainly because its a 4000.00 lens, or an 800.00 lens. Some camera stores are leary about people buying a lens, and returning it a week later.

The right glass is the most expensive part of photography, and in my opinion, renting it first makes a lot of sense.

I think renting lenses is great. Especially if you only need for a trip or specific project. However, if you are looking for a Canon L lens I would recommend buying a used one and sell it when your done. I did this last year for the 100-400 L that I knew I needed for a month. I bought it used (from http://fredmiranda.com) and sold it a month later for the same price. It ended up that the cost of shipping one way was cheaper then the rental fee for the same time period.