Finally decided to pop for one of these, and ordered one from a reputable HK dealer. Lens arrived, and initial impressions from handling the lens were good.
However, first images did not impress, and so I set up comparative tests with other lenses covering equivalent focal lengths, using LiveView on a 450D, with tripod, MLU and remote release to eliminate variables as far as possible.
Results were very interesting. Centre performance was not stunning, but OK. The corner performance (even on a 1.6 crop camera) was however far from acceptable, even stopped down, at focal lengths of 28mm or greater. At 35mm, the IQ in the corners with the lens wide open were nothing short of abysmal -- the quality was so poor, it was visible on the rear screen. Comparative tests with an EFS 18-55mm (the first version) at f5.6 and f11 showed that the kit lens trounced the L.
On the positive side, at 20mm, the L was better than my 20mm f2.8 USM, so it was not all bad.
All in all, I wasn't impressed, and attempts to find out if the lens could be adjusted in the UK by Canon or its service centres failed, so I decided to return the lens for a refund.
If I buy this lens again, I'm going to perform tests on a copy in the shop before putting my money down. I may try out the new Zeiss 18mm f3.5 when I can, as an alternative.
There certainly are bad copies of this lens around, and though I own many (17) EF lenses, many of them L, I'm going to be more cautious about buying Canon glass in the future. At this price point, rubbish lenses like this should not be in the supply chain.