I purchased the 28-300 L about 3 years ago. It was a used model I saw advertised on e-bay. It was almost new but was not advertised very well. So I managed to get it for £1005 which was a very good price at the time. I loved the focal length and I was very satisfied with the results. It was perfect for holidays and a good all rounder goto lens. Whilst this is not a particularly fast lens the clarity throughout the zoom range is impressive with little vignetting at the extreme end. I did not find the weight or size of the lens a problem as I mostly shoot with a monopod. The IS works very well. Please note you cannot use the 1.4x or 2x extenders with this lens.
Unfortunately what annoyed me with it was that a little bit of paint on occasion would come off the barrel due to the sliding pump type action required to operate the zoom and this fleck of paint if you were not careful would be sucked into the lens itself. So to prevent this from happening you don't tighten the tightening ring as much. But over a period of time the barrel would show much paint loss due to wear. For someone like myself who likes to cherish and look after their lenses this unsightly paint loss although you could not see when the lens was not extended was a factor that helped me to decide to sell it. Eventually the Tightening ring (used for eliminating zoom creep) would not tighten any more and the lens would move freely up and down the barrel.
Although this being just cosmetic issues I decided to sell it after over 2 years of use. I put it on e-bay and let the auction run and to my surprise I sold it for £1100. If these problems had not of happened I would still of owned the lens today as the pictures I got from it I was very pleased with.
But as it turns out I'm glad these problems did happen because I replaced this lens with the 70-200 Mk2 and WOW..... what an incredible improvement. I just love the f/2.8.
Hope the above all makes sense.