[drmaturin]

I advise caution:

Based on the reviews, I bought a Sigma 150mm last fall to hopefully give me greater working distance than my Micro Nikkor 105 D (pre-VR), especially since it was supposed to behave well with the Sigma 1.4x converter I already had.

I immediately tested it against my Nikkor. The Nikkor was so much better in all tests (conducted with and without flash and always on a solid tripod.)
I used feathers as the macro test subject because the level of detail is incredible right across the frame. i used a brick wall to test the long distance.
The Sigma images had poor contrast, an ugly, yellowish color cast, and weren't nearly as sharp as the Nikkor. The difference was huge. I couldn't believe it at first. To top it off, the Sigma underexposed a lot of the flash macros. I must have gotten one of the bad copies that seem to regularly slip past Sigma's Q.C. dept. I retested two more times, and did some real world stuff. Nope, the lens is not good, at least when compared to the Nikkor.
I couldn't return it, having purchased it at one of the grey-market shops in Seoul (lesson learned) and went on a long holiday before I could send it in for servicing, which I will now do.
Long story, but it shows that reviews are not always indicative of the actual product that you may take home.
Obviously there are tons of happy campers out there with this lens, but not me.
You should also know that it's quite a fat and heavy lens, and with no VR hand-held at that magnification become problematic in lower light. Flash and/or high iso will be called for.
Personally, I'd get a shorter f.l. lens for portraits any way. There are lots of lenses lighter, cheaper, and just as sharp as even a good copy of the Sigma. Or that Canon zoom. You might miss the bokeh of 2.8, though.