Canon EF 70-300mm f/4.5-5.6 DO IS USM

A month ago Canon lent me two lenses, one of which was the EF 70-300mm f/4.5-5.6 DO IS USM, a lens which I have a previous version, the old EF 75-300. I gave it a go in the week I had it, but my feeling were absolutely mixed. Let's start by saying that lately I have gone really wide, and already the 85mm f/1.2 I received was quite too long for the many things I normally do, nevertheless I gave the DO (Diffractive Optics) a try, comparing the results with my old lens.
Feeling When closed, the new Canon lens is quite compact and heavy. It seems to be well built and thanks to the diffractive optics is not extremely long. Being used to the 75-300 (the non-IS version) I was quite surprised by weight as it was much more than the previous one. I have never used the IS version that was probably heavier and bigger than the version I own, so I will compare the last version with the first version. With the lens completely retracted the new version is some inches shorter, while when the lens is extended at 300mm the difference is practically nothing. The weight difference, on the contrary, is huge: the old 75-300 is very light with a very "plastic" feeling, while the 70-300 DO is strong, heavier than I had expected. The new lens seems really solid and well built, but to be just f/4.5-5.6 the weight is a little bit too much in my humble opinion.
Photos Don't get me wrong: the Canon EF 70-300mm f/4.5-5.6 DO IS USM is a very good long lens if you do not need a wider aperture. The focusing system is fast and silent, the lens extends easily and the photos produced with it are sharp. If you know the drawbacks of the lens and you can cope with them (mainly weight, aperture and price), this lens is a little jewel. It is important for me to remind you that this is not a L lens, but the quality is still superb!
Price Even if the quality is superb, the Canon EF 70-300mm f/4.5-5.6 DO IS USM is NOT a L lens, but the price may suggest so. We are talking of a lens that costs more than £1000. Seriously. Personally I would NEVER pay such a price for a lens with f/4.5 of maximum aperture, even in a moment in time where the lens has gone up in price like gold. Last year I paid my 75-300 less than £200. Indeed it is not a DO lens, it is not a IS lens, it is a little bit longer, but it is lighter, thinner and much lighter (and with a better maximum aperture of f/4).
Is it really worth it? Clearly every conclusion is absolutely personal, as mine is. IF (and it is a big if) the Canon EF 70-300mm f/4.5-5.6 DO IS USM would cost £500, I would say that it is a Good Lens, probably worth its price. Probably. Issue is that we are talking about a lens that costs twice as much. As always you have to analyse your requirements, what you would use your lens for and what negative factors you are going to accept. It is a simple matter of balancing pro and con.
Would you buy it?