I used to have one, and I don't now, so I guess that says something. But I didn't get rid for reasons of poor optical quality. I thought it was fine in that regard, and I always looked quite critically at the photos because so many people diss that lens, looking for problems that are only noticeable if you go looking for them. Unfairly IMHO.
I bought it because of its size, and at 70mm it is indeed very compact. But it's not at 300mm, and with a hood it's as long as any other 300mm. It's small for carrying, but not discreet to use. It also suffers zoom creep, which really annoyed me. And it's heavy - 100g more than the regular 70-300 IS.
Apart from taking up less space in your bag, the normal 70-300 IS beats it in every respect.
Edit: crossed post. I now have a 100-400L, and also 70-200L 4 IS. If size/weight is not a problem and you want reach, then 100-400L is the one to go for. The 70-200L 4 takes a good quality 1.4x extender quite well (Canon or Kenko Pro) as does the 2.8 version, but very few lenses take the 2x very well - not for regular use anyway.
If you're uncertain, I would hire a couple of lenses from StewartR on here
www.lensesforhire.co.uk and compare them for yourself. It's taken me a couple of years and four different lenses to arrive at what I have now. Expensive mistakes
