Embarrassing U-turn...
Well, this is very embarrassing but I'm going to have to take back what I said about this lens. One particular post I read (elsewhere) talked about focus shift, and that rang a bell because some of the close up shots I took yesterday were mysteriously out of focus - I thought I'd focused carefully but then when I got them home they were wrong. Specifically they were back-focused. I just put it down to pilot error.
So I've just conducted a simple test - ruler on the table, camera on a tripod, very oblique angle, aim at 10 cm on the ruler, focus carefully using 10x Live View, and shoot at f/4, f/5.6/ f/8, f/11. First in non-macro mode - and sure enough there is
very serious focus shift. I mean, at f/8 it is so bad that the intended focus point (the 10 cm line) is actually out of focus. So I tried again in macro mode, expecting the problem to vanish. Shooting as I was from about 43 cm, the overall IQ was noticeably higher, but the focus shift is still there.
This is the non-macro mode test - click the image to see it full size. You can clearly see that the left-hand (f/4) image is focused on the 10 cm line; f/5.6 is focused at 10.7-ish; f/8 at 11.5-ish (and notice how the 10 cm line is well out of focus now; f/11 is harder to judge but say 12 cm. (NB if you are unfamiliar with this - the aperture is the only change between shots - there is no refocusing.)
There one more part to the story which I'd better tell. This is actually my second copy of the lens. The first one went straight back because I couldn't get a sharp image from it. The supplier, who I won't name because this is not their fault, was very helpful and sent out an immediate replacement. Great service actually. However it arrived on Monday and due to work commitments I didn't use the lens until the weekend. Now I realise why I couldn't get sharp images from the first lens!
It's a tragedy as far as I am concerned. The specification of this lens is exactly what I needed, but it doesn't work - it's not fit for purpose. No wonder Canon is selling them off cheap. Well, this one is going back.