I got my first DSLR about a week ago (600d) + af-s 18-135mm kit lens, but I want another lens with greater focal length. So I thought about this lens:
CANON 70-300mm f/4-5.6 IS USM
I own that lens. It is, indeed, a solid performer at that range for the money it commands. You'll find a lot of grey area when evaluating whether it's the right next lens for you, but I'd certainly encourage you to stay away from the 75-300; it's just not a good performer when stacked up against low/mid-range lenses like the 55-250 or 70-300.
You may find this lens to be "ok" for wildlife, but it's pretty likely you'll be wishing for even higher quality if you do any serious wildlife photography. Unfortunately, just as with sports / action photography, high-end glass pays serious dividends in wildlife photography. There are a number of alternatives that offer either higher quality or better reach than this lens does, but as you go down the list, you'll start to see that the 70-300, while not the sharpest or the longest, is a pretty decent compromise, and a fine value compared to some of these other kits:
At this point, you might begin to appreciate that (1) wildlife photography can get pricey in a hurry, and (2) the 70-300, even with its imperfections, is a pretty decent value lined up against some of these alternatives.
As is usually the case, your best bet is to lay your hands on a copy and take it for a spin, if at all possible. There's nothing like real-world usage to help you understand the performance envelope of a piece of equipment like this.
FWIW, here are a couple of photos from this lens. This first one is wide-open (f/5.6) at 300mm:
Sharpness is fair at best here. Bokeh is ok, but nowhere near the creamy results you'd expect from better lenses. Next, here's one stopped to f/6.3 at 190mm. Since this lens isn't super-fast optically, I'm not above using a bit of fill flash to help out a shot like this:
Sharpness is better when the lens is stopped down a bit, but not "L" sharp, of course. Finally, an action shot (although not wildlife) shows good autofocus performance, provided you've got sufficient light. This lens will focus-hunt when light is dim. This shot is 170mm at f/6.3.
For carrying, by the way, I use the Lowepro Lens Case 2S, which fits the lens & hood (reversed) very well.
Despite being as cheap at it is and variable aperture, the Canon 70-300mm F/4-5.6 IS USM is a pretty good lens between 70 and 200mm. It is quite sharp at those focal-lengths.
It shows virtually no vignetting and very little distortion. From 250 to 300mm it is rather soft and no amount of stopping down will help you unfortunately.
Since you intend to shoot wildlife, you will certainly want to shoot long, so you should consider the L version instead which is sharper at the long end and particularly so stopped down to F/8 or so.
Keep in mind that neither of these lenses is fast and if you shoot wildlife in action or in low-light as is often the case on Safari, you will get blur from your subject's motion. For one-off events, you may consider renting since fast & long lenses are very expensive.
I used to own the non-IS version of this lens, got it as part of a kit with the EOS 350D! Although it did me fine in the early days of getting into DSLR photography, it never really lived up to my expectations as regards to IQ. I figured maybe it was down to the older technology of the 350D, with it's 8mp sensor and not very good AF system, but when I eventually upgraded my camera to the 7D, I noticed no improvement in the image quality from the 70-300. Images were the same as before with regards to softness, etc.
Eventually I got rid of the 70-300 and replaced it with the EF 100-400mm f/4.5-5.6 L IS USM, and IMMEDIATELY noticed a huge difference. The two are worlds apart.
It's so versatile from wildlife, to aviation, to lunar photography. It also makes for an interesting portrait lens believe it or not (as long as you can get far enough back).
If you can save a little longer, I'd really recommend trying to pick one of these up. I can't recommend it enough.
In the interests of fairness I should point out some potential pitfalls of it though. The main one is the slow f/4.5-5.6 aperture, which, in low light, may mean shutter speeds sufficiently slow to introduce blur. The other thing is the push-pull zoom mechanism which is different to any other lens I've used. But this is a minor issue and you get used to it very quickly.
Have a look on eBay - that's where I got mine second hand - and at a bargain price too (£820)...
