I personally think that primes are the way to go for portraits because I like a very narrow depth-of-field. I own a 85mm 1.8, and it is almost too shallow- I often need to turn it up to focus correctly. If you use canon, look at the 24-105 f/4 L series. I got one used, and I am very impressed with its IQ. Very sharp, and with IS, it works fairly well in low light. it is somewhat heavy, and somewhat slow (f/4 fixed) but is becoming my favorite carry lens.
