Ohh....I just gotta jump in on this one!
I have the Tamron 70-300 LD and for the price
it is, in my ever so humble opinion, one of, it not the best lens in this price range. I got mine used on Ebay for $90 and it's really worth several times that. My only gripes about this lens are that at f/4-5.6 it is a slower lens (obviously) and it is also a slow focusing lens. Also because I shoot hand held so much, I wish it were image stabilized as well. On the other hand as I've said many times before, if this lens were an F/2.8 "VC" (vibration control) and had faster focusing, I'm sure it would be over a $1000. The sharpness of this lens though is fantastic even at several times the price.
Here's some examples shot with the 70-300mm with my Canon Rebel XT...
As you can see, this -IS- a sharp lens. Several of these pictures were taken in lower light situations too (shade or overcast days) so even the speed of the lens isn't that
big of a deal in many cases.
Now with that said, as Luca mentioned these lenses are rather slow at focusing. It seems to be a hallmark of Tamron lenses in general. However, I've played with the Canon 70-300 IS and it doesn't focus that
much faster than the Tamrons. In fact I thought the focus speed was pretty similar. The Canon lens is quieter, but it still focuses rather slowly and does tend to search in low light.
I do however disagree with Luca about the print quality that you can get from the Tamron lens. -ALL- of the shots above have been printed as 8x10's and the prints are as good as you see here and again I'm sure compare with that from the $550 Canon lens.
This next shot was taken really close to sunset in heavily shaded woods...in other words, it was fairly dark outside and I did have to have the ISO on the Rebel cranked up to 800. As such, the shot did get rather grainy, but that was from the ISO of the camera and not
the lens. The shot did require a good deal of noise reduction in Photoshop but again as you can see, it's not a bad shot and the quality is certainly decent all things considered (and again this made a pretty nice 8x10 print)...
Had I of been using something like a Canon f/2.8, I'm sure I wouldn't have had to do nearly as much clean up on the shot. Of course the same could be said of the camera as well...had I of been using something like a 5D or one of the better full frame sensor cameras with an ISO that will shoot in total darkness, the noise probably wouldn't have been an issue either. But again here, we're talking about a significant price difference. In the case of the lenses, we're talking about a $1000+ lens versus a $90 lens and for the that kind of price difference...well, you can decide for yourself.
Very simply, as with so many things in photography, this really comes down to compromise. For people who can drop a couple of grand on good glass, I would certainly recommend getting those "L" lenses all the way around...they're sharp, fast focusing and do great in low light (which is why they are "pro" lenses), but I get the impression that you are in the same boat as I am where $600 can simply be a HUGE investment. Unfortunately in this price range you don't have nearly as many choices and very often you have to learn to just deal with it and do the best you can.
Now to compare the lenses you've mentioned... This again is strictly my ever so humble opinion, but barring the issue of the Image Stablization on the Canon lens, I really do believe that you are going to get very
similar quality as far as your images go from either the Canon 70-300mm or the Tamron 70-300mm. They are both quite comparable in regards to sharpness. -IF- they were both the same price, I'd go with the Canon simply because it is image stabilized...for shooting hand held, IS is a real plus to say the least. But between just these two lenses even new, you'll have to ask yourself is that Canon IS really worth the extra $300
...and only you can decide that. The image quality is very similar, but the IS is going to give you a sizable advantage.
Now lets talk about that Tamron 28-300 VC. While I haven't used one of these personally yet, I've been reading the reviews also and they are getting -very- hi marks when it comes to image quality. Also Tamron is the ONLY company right now doing 3 axis image stabilization...even those pricey Canon L lenses only use 2 axis stabilization. In fact the only negative comment that I've seen consistently is that like most Tamron lenses, they focus rather slowly (go figure). So between the Tamron 28-300 VC and the 70-300, you'll have to ask yourself is that image stabilization really worth the extra $300. However between the Canon 70-300 IS and the Tamron 28-300 VC...you're talking more focal length/zoom -AND- better image stabilization at roughly the same price. And based on my other Tamron lenses as well as the reviews and Tamron's reputation in general, I'm sure the image quality is again going to be very comparable if not better than the Canon. Ya don't really need to be a rocket scientist to figure that one out! LOL!
Now one last thing I would like very much to add here. Please understand that this isn't aimed at any person in particular and I certainly don't mean any offense towards anyone but please remember that there are a lot of people who I believe are a little obsessive about "brand name" when it comes to lenses. At one time
it was true that a Canon or Nikkor lens would outperform a comparable after market lens such as Sigma or Tamron, but that's simply not true any more. Tamron, Sigma and Tokina all make some truly great lenses now that are as good as, if not better lenses than their brand name counter parts when it comes to image quality. Further more, I don't think a lot of people really take the price difference in to consideration. Yes, a $1500 Canon L lens is going to focus faster than a $150 Tamron or Sigma lens...but that should be
a given. That $550 Canon 70-300 doesn't really focus that much
faster than a Tamron...maybe it's a bit quieter, but it's not really much faster at all (at least not the one I was playing with at Circuit City!). -Any- lens in this price range is going to have a tendency to "search" in low light situations...tis the nature of the beast in this price range. However when you actually compare apples to apples here, you'll see there isn't really too much of a difference when it comes to quality and when it comes to price, Tamron and Sigma blow Canon and Nikon away every time.
I think it's all a matter of perspective really. If you want a lens that performs like a $1500 lens, than buy a $1500 lens...it's as simple as that. In the price range that we're talking about though, while I hate to be the odd man out (well...not really) for the price difference, I'll take the Tamron every time. To be honest, the primary reason that I've personally even considered the Canon L lenses is simply because Tamron doesn't make something in these focal lengths that is a fast focusing f/2.8 IS. If they did, I'm sure the lenses would probably be at least a few hundred cheaper than Canon (or Nikon) and that that's probably what I would choose.
Lastly, I have to agree with Bob...regardless of Canon vs. Tamron, etc., a 300mm lens is still going to limit you to a large degree with birding. Unfortunately, there are no easy (or cost effective) compromises here. You can get something like a 2x converter, but with these slower lenses, you're going to have even more of an issue with light and focusing. I have a 1.7x converter and while it can do a decent job, it's a real muther to use with my 70-300mm. The other option is to start looking at the really expensive lenses. The cheapest (that I've seen) is the Canon 100-400mm (which may still be on the short side for you) for around $1500 and from there the sky is the limit when it comes to price ($6000-$10,000 in some cases). Of course, you could also just do what Bob and I do and just get closer to your subjects by shooting them at zoos and nature centers
Alrighty...I hope that helps! If you're considering that Tamron 70-300mm (or the 28-300mm VC) and you don't mind the exceptions and addendum's in the fine print as it were, look at my pictures....as they say, the proof is in the pudding. You can do a lot with that 70-300mm if you're willing to work with it a little and understand it's short comings.
(...ya know, I'm starting to think the folks at Tamron should really start compensating me for all these endorsements! LOL!!!)