rentglass.com, borrowlenses.com, zipglass.com, etc. all seem to be pretty reputable places to rent lenses. If you type in "Canon lens rentals" to Google, you should come up with a pretty solid list of places to rent and if you want to check our if they are legit go to resellerratings.com and see what people have to say.
As far as whether or not you should rent a lens, that's all up to you. Would I rent a $200 lens for a trip? Probably not, I'd come up with the money and either buy it or buy it and then resell it as Tom suggested. Would I rent a $750+ lens? Definitely. With coupon codes and promos, the lenses that I have rented have always been pretty cheap and I have gotten a feel for whether I wanted to buy it or not.
With the 80-200 f/2.8 that I rented in December, I was able to shoot with it for two weeks before deciding whether I wanted to buy it or save for the 70-200 f/2.8 instead. It was $100(ish) well spent and was much more convenient than having to buy a 80-200, have $900 of my money wrapped up and then return it and wait to get my money back or resell it in hopes of turning a profit.
Good luck and have fun!