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:
- 70-200 f/4 -- comparable in price to the 70-300, but without IS. Widely considered to be a great value in the "L" lineup, with very good sharpness. Missing a fair bit of reach compared to the 70-300, though, and no IS (which is typically a big deal, but may not impact you quite as much for wildlife photography if you work to keep your shutter speed up).
- 70-200 f/4 IS -- Adds IS, but you're approaching twice the price of the 70-300, and you're still missing 100mm of reach.
- 70-200 f/2.8 + 1.4 or 2.0 TC -- The faster f/2.8 lens works better with a teleconverter (which gets your reach back), but obviously adds cost as well.
- 70-200 f/2.8 IS + TC -- a nice, versatile kit, and a big hit to your wallet.
- 100-400 f/4-5.6 -- popular among wildlife photographers. Not super-fast optically, but a very useful range. About 3x the price of the 70-300.
- All the rest of the big Canon primes are also staples of pro wildlife photographers, but these lenses start to get really expensive.
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.