I bought my EF 70-200 f4L in May 2007 for 430, after much checking of web reviews and prices. Normally I buy 2nd. hand, and have had very few problems buying a dozen lenses over more than a decade of using Canon. I prefer and use primes on my 5D, but after taking my EF80-200 f4.5-5.6 to the Dolomites, I felt that a quality zoom should give better results. In view of the mixed reviews I found on the web, this time I bought new and returned my first copy as soon as I could. The very poor sharpness on just the right hand side when close to the 70mm setting jumped out at me - careful tests simply confirmed how bad it was. It got better away from 70mm, but this performance made my old 40 EF80-200 f4.5-5.6 look stellar! (I've had decent A3 prints from it.)
I requested a 2nd. copy and although (now knowing where to look!) I can still see some problems with this copy, I decided that the performance was acceptable, and nearly a year on that was the right decision.
It covers a useful range on my 5D, focuses fast and pulls in detail in urban landscapes extremely well. It's light enough to live in my bag (I guess the f2.8 version would have lived at home.), and I'd recommend it.
Standard of Build? Similar to my other L lenses - feels solid, but so does my EF 85 f1.8.
Do I use it for birding? Not intentionally. I use my EF300 f2.8L IS with extenders for that.
My low rating is simply that I object very strongly to doing Canon's QA on an L lens, or on any product. (This is only my 2nd.lens purchased new - my copy of the EF 100 f2.8 macro USM also needed early repair under warranty as it started coming apart.)