Perhaps my biggest reservation about the 35IS relates to the 40mm pancake. Comparing them:
- my feeling is the 35IS is a little sharper, but there's not a whole lot in it
- my feeling is the 35IS has slightly better colour and contrast, but there's not a lot in it
- my thinking is the 35IS has slightly nicer bokeh, but there's not a whole lot in it
- the 35IS has noticeably faster and quieter AF - but that's not to say the pancake is bad in those respects, so query how much difference this is likely to make in practice (no doubt it depends in large part on what you're shooting)
- the 35IS feels more substantial and hence makes you think it may have better build quality - but I have no idea whether, in reality, the 35IS is likely to be any more durable. (In this case I strongly suspect it is likely to be more durable than the pancake, but all the same I get sick of reading lens reviews which seem to equate weight with build quality, and conclude anything light weight is lesser quality. Isn't that like saying something made of steel is always a higher build quality than something made of titanium or carbon fibre?)
- of course, the 35IS has a one stop aperture advantage (which you'd rather have than not), and IS (worth at least another 3 stops - which allows you the choice of longer shutter times or lower ISO)
- the 35IS has 67 filter thread, which means you may already have filters you can use on it (unlikely with the pancake)
- the extra 5 mm of width (in the focal length) is noticeable on the 35IS but again, it's not very different - and to the extent there is a difference, each has its pros and cons
- much better focus ring
Weighed against that, the 35IS is around 3x more expensive than the pancake, substantially larger and over 2.5x heavier (even if it still ranks as a relatively small and light lens in the bigger scheme of things).
So, my question remains about the value of the 35IS compared with the pancake. The 35IS clearly offers more flexibility in that if you want to be able to handhold shots of still subjects in low light, the 35IS is way in front. If you're subject is moving, the 35IS still has the advantage but the gap is much closer. If you have enough light though, the 35IS's IQ advantage doesn't seem to be that great really, so if you're using it in well lit conditions, it's less clear to me whether that advantage is worth the extra cost/weight/size.
Put another way, and taking the position most people can hand hold a 35mm or 40mm lens at 1/60 second in most circumstances (noting the debate in one of the other message threads about whether 1/focal length is a suitable guide or if these days it's closer to 1/double the focal length, I'll use a compromise here), if you'll use the lens in circumstances where you want to handhold shots at shutter times longer than 1/60 second, the 35IS offers a clear advantage (and more so as you get into the 1/30 second range and longer). Otherwise, though, you're paying quite a lot more for only extra stop of shallow depth of field ability, and a few other things which may be of limited practical value to some people (eg the better AF).
If Canon made an error with the pricing I think it was with the 40mm pancake. The lens is just too good for the price ! Canon could easily have 'crippled' it in such a way as to stop people like myself buying it; just give it a plastic bayonet mount, and that's the likes of me out of the frame.
Regarding the statement "there's not a lot in it', I'd say "There's nothing in it' !