After reading on this site numerous times about Lensrentals.com, and the owner, Roger, having a number of blog articles linked to on here as well, I decided to go with them for my first ever rental. It arrived yesterday!
I decided to test out the new Canon 24-105 f4L ii and they had it available! Unlike 'the other guys' mentioned previously that make you pay for rental time while it is in transit, Lensrentals gave me the full 7 days to use the lens. I didn't hear Fedex ring my doorbell on Thursday as I was on the basement doing wash, so I got it yesterday. The email they sent me an hour or so after I got it says I don't have to get it back to Fedex until next Friday. I get the full week! If new mark ii's don't become available in time for my early May vacation, I will likely rent the lens again!
By the way... the mark ii blows my MFA'd mark i out of the water on IQ in my very informal tests from my back porch. And yes, the extra 5 oz in weight is noticable but acceptable. The slightly larger size, not so much.