I got the notification from B&H that the Canon tripod collar was now available for the 70-300L for $190. After using this lens for the last 9 months, I've decided to pass on getting the collar. I don't think you need it.
I also thought I'd want the collar for the 100L macro and purchased it just to have it sit in my camera bag. Don't want to make the same mistake twice. I'd also not buy a third party one if i thought I'd need one.