For the money you can get one off Craigslist for, the 28-135 is a good lens. It's mid-grade build quality is considerably better than most kit lenses. It's got relatively fast, quiet, accurate USM. The I.S. on it is an earlier or simpler version, but still effective for a couple stops. Image quality is actually quite comparable to 24-70L and 24-105L.... You'd have a tough time telling apart 8x10 prints made from each. 28-135 is also reasonably close focusing. A couple minor issues... it's common for the 28-135 to have "zoom creep". And sometimes the barrel is a little sloppy (seems slightly loose). But neither have much or any effect on the lens' capabilities.
One thing since you mention portraits... The 28-135 isn't a large aperture lens, heck it's f5.6 out at the 135 end. A lot of time for portraits its nice to have f2.8 zoom or a prime that has even larger apertures, for nice background blur effects.
I also like the focal length for a walk-around lens, on a crop camera it's a standard to moderate telephoto. You might want to compliment it with something wide, such as Canon 10-22... or Tokina 12-24/4, which I think is another really good value lens.
Eventually for wildlife you might want something longer.... Canon 100-400, Sigma 120-400 or 150-500.