Canon 70-300 F4-5.6 IS USM.
When I originally wrote about this excellent mid-range zoom I did so with reference to its expensive, older big brother. I'll talk about my experiences with the 100-400 elsewhere and limit this to talking about the 70-300 instead. If you don't need the extra reach and can live with stopping this lens down a third of a stop to F6.3 and you only need 300mm its a no brainer. This is a superb performer for the money.
First off, the good. What you get is excellent performance in a compact package - learn to workaround this lenses limitations and you'll come to love it. Image quality is excellent - although it does require stopping down to F8 especially towards the long end - with great contrast, sharpness and colours as a result. This is probably a result of the inclusion of a UD element (normally only found in "L" series glass). As you'll see looking through my galleries a great deal of my favourite images were taken with this lens - in fact I was surprised by just how many! The autofocus is fast and accurate. The image stablisation really works - especially for someone like me who generally eschews tripods out of a combination of laziness and general dislike of them. Let me say that again - the image stablisation really works. It makes this lens. Bokeh aside, I've been hard pressed, at F8, to tell the difference between this and and a 100-400 IS. Seriously. I'm not alone in this either. The colour and contrast granted is slightly better with the 100-400 - but you have to look hard to see the difference. Of course 300mm isn't as much reach as 400mm ...
I'm cheating a little here as I deliberately picked an image taken with the 70-300 where the background didn't impact on the bokeh. You can see the larger images in my gallery - the one taken with the 70-300 at F8 is on the right - the one taken with the 100-400 at F5.6 is on the left. They were both handheld in very similar conditions on the same day. I don't think there's a lot between them, personally. If its important to you there's a slight difference in focal length - 340mm vs 300mm - between the two images.
Here's a 100% crop reduced by 50% to give you a taste of the contrast and sharpness :
Another example - a juvenile robin this time at F7.1, 300mm, handheld with no PP sharpening applied :
100% crop round the eye :
There are a couple of disadvantages as you'd expect - first off its a comparitively slow lens as it requires stopping down for best results, especially at the long end. I find that the IS in combination with a higher ISO setting to give a faster shutter speed helps to offset this. The second is that the front element rotates when focussing - which means using a polariser is maddening to the extent where I can count on one finger the number of times I've used one. Also there is no full time manual - which is a pain if the camera focuses on the wrong thing - or you haven't got time to pick a different autofocus point. The third and biggest is obviously that you can't use this lens wide open and expect decent shots. I've found you need to stop down to F6.3 or F7.1 to get a pin sharp image every time but that's almost wide open. Not bad for a "non L" lens.
The build quality is a lot better than that found on the frankly pointless Canon "cheapie" lenses - and will more than adequately stand up to normal wear and tear. Its not up with Sigma's EX let alone Canon's "L" finish, though with some parts - notably the extending focus barrel and manual focus ring - feeling flimsy and plastic. Overall it does feel solid with no discernable creak or give. Being that this is a Canon non "L" lens you'll need to buy the hood separately. I've not suffered from flaring - except when shooting into the sun at sunset. A hood wouldn't have prevented that - so I've not seen a reason to pick one up.
Like many owners of this lens I bought it intending to use it as a decent stop gap whilst I saved up for an "L" series zoom. I have since replaced it with the 100-400 (another stop gap). The 70-300 isn't compatible with Canon Teleconverters - although it does seem to work with third party ones. I've no experience with this so I won't add any thoughts on the matter. There are times when you want a compact, carryable package and unlike the heavy white "L" monsters the 70-300 offers this. The image quality stopped down is simply excellent for the price and its certainly ideally sized when weight and space are an issue. You really can't underestimate the benefit of image stabilisation either - and I'd personally rather have the extra reach and IS over a shorter non IS "L" zoom.
As I've said at F8 I couldn't see any discernable difference between the 70-300 and the 100-400 in terms of sharpness. If I looked hard I could spot a very small difference in contrast and colour. I stress small - nothing like the knockout improvement I see when I use the 24-105 though. Of course, the "L" lets you shoot sharp wide open. Being limited to F8 to get the sharpness with the 70-300 meant occasionally that I get "messier" backgrounds than I would otherwise have liked. Some of the in flight bird shots in my galleries suffer particularly from this - and the L lens would defintely have improved things here. Lets be honest - the bokeh with the 100-400 is better wide open than the 70-300 *but* neither are a patch on Canon's fast, expensive long super primes.
So, having moved on to 100-400 do I miss the 70-300 ? Initially I would have said yes. Why ? The size. However, I'm now used to using the 100-400 and I wouldn't pass over the extra 100mm or the ability to shoot sharp wide open. For my style of photography the extra reach allows me to get pictures I would otherwise miss - and not having to stop down to F8 all the time gives me much greater control.
So in conclusion - this lens offers great image quality for the price and is a brilliant travel or walkabout lens. Having used it alongside the 100-400 there's no doubt in my mind that the 100-400 is clearly worth the extra money - if you can't live without the extra reach and usable faster aperture. In terms of bang for buck the 70-300 seriously delivers and as such I wouldn't hesitate to recommend it.
Whilst I appreciate my comments are mainly subjective you can see some technical data at Bill Castleman's site.
Pros : Comparitively compact and lightweight, superb IS, excellent image quality, acceptable build quality for general use, doesn't stand out.
Cons : Not an "L" lens, build not up to serious misuse, no distance or depth of field scale, soft wide open at long end, no FTM. F8 required for sharp images - bokeh dissapointing in crowded backgrounds.
Conclusion : Don't need 400mm and want to shoot mostly handheld ? This is ideal.