This is going to be just another five-star review of this lens, but this is a lens that completely deserves it.
First off, if you're like me, you were drawn to this lens wondering what on Earth you'd ever want a lens that's intentionally soft for. Well, "soft focus" is not the same thing as being out of focus. No, even with this lens's soft focus features at maximum, the photo is still razor-sharp, there's just a soft glow coming off the bright parts of the image. This effect is controllable with both the "Soft Focus" dial (0, 1, or 2) on the lens, or through the aperture value. Soft Focus 2 and f/2.8 is the "softest", and decreasing the aperture size and soft focus value both lessen the effect. Additionally, you can shoot with the soft focus value not "clicked" into a number, but between the numbers.
But now, enough about that feature, since you'll likely rarely use it. Apart from stylized portraits and nature shots, you'll probably keep the Soft Focus dial locked into zero. So then, what is so great about this lens? The answer is simple: it is an amazingly sharp f/2.8 telephoto lens. This lens is the smallest and lightest f/2.8 telephoto lens on Canon's current lineup, but this is not to say it is constructed cheaply. It is a very solid build, with a metal camera mount, and thick rubber hood built-in. The dials and controls are an older Canon style, so the focus ring isn't as smooth or easy-to-use on the newer lenses, but it still works like a charm.
But let's get back to the part about how amazing this lens is. I shoot full-frame, so the "fringes" of lens's image areas are especially important to me. But this lens doesn't miss a beat -- even at f/2.8 it is sharp, focused, and free of distortion to the far edges of the frame. Drop it down to f/4, and there's a good chance this'll be the sharpest lens in your collection. Colors are always bright, out-of-focus areas are always smooth, and this lens is, without a doubt, a top level performer.
So why is this 135mm f/2.8 not decorated with a red ring and an "L" in the name? The lack of Canon's USM technology means this lens lacks the 'full-time-manual' feature. Despite this, the lens still focuses quickly and quietly. Another feature you won't find here is Image Stabilization, but with how light and compact the lens is, you won't miss that. And finally, the minimum focus distance is 131cm, which while not atrocious for a telephoto lens, is something you'll find yourself bumping up against while shooting.
But to wrap this up, none of the "drawbacks" of this lens matter when considering how amazingly sharp and versatile this supposed "soft focus" lens is. Canon's literature talks up this lens's portrait abilities. They are very true, once in your camera bag this will become your portrait lens. But you'll find yourself popping it onto the camera just about every opportunity you get, since it is such a joy to shoot with.
This lens is such an awesome hunk of glass, I have zero regrets adding it to my collection.