Okay, any serious photographers can stop reading right now... Due to my finances (and my family finance minister), I'm going to be talking about some "bottom of the barrel" lenses here. ;)
I picked up a new Canon EF 75-300mm f/4-5.6 III lens yesterday. I was actually looking for a Tamron AF70-300mm F/4-5.6 Di LD Macro 1:2 lens, but the Canon was a bit cheaper and it was in stock at my local dealer. I figured they would be about the same, with the exception that the Canon wasn't a macro lens. I had the guys at the shop look up the specs on both lenses and the difference in focusing distance between the Tamron and Canon was only about a foot, so I figured I could live with the Canon. Unfortunately, the Canon ain't all I thought it would be. :(
There's nothing wrong with the optics that I can tell. The DOF is very nice, which is what I was looking for. The magnification is also pretty sweet, especially with the 1.6x magnification factor I get from using the XTi/400D... that basically makes the 75-300 act like a 120-480mm lens. But...
The mechanics on the Canon lens are REALLY bad! The Zoom control is near the base of lens and the focal ring is near the front. This is basically backwards from most lenses. Not that big of deal, but still "odd". That's just an nagging issue to me, not a deal breaker. The major issue is the zoom control. It's "notchy". It basically feels like the mechanics in the lens are binding up. This means that when zooming, the action is "jerky". It's impossible to do a smooth zoom. This is most telling when you're trying to get to an exact focal length. With the Canon lens, I've found that I have to overzoom, then pull back to get the zoom where I wanted it. It's a bit of a pain to deal with on a regular basis. I did some Googling on this lens and found that I'm not the only one with this issue, so it's obviously a "design feature", not an error with the manufacturing.
I should also mention that this lens is quite bulky and heavy. It probably weighs more than the XTi/400D does.
The final straw is the focusing mechanism. It's an AF lens, with a switch to convert to manual focus. Unfortunately, the AF is REALLY slow, has an incredibly hard time tracking stuff and doesn't allow you to manually correct it without throwing the switch (this appears to be the way that non-USM Canon lenses work - get a USM version if you want to override the AF). The length of time it takes to autofocus when zoomed is ridiculous. So...
The Canon lens is going back and I'm going to get the local shop to order the Tamron for me. I'll post up a comparison when I get the chance.