An absolute must. My wife and I did this for a whole afternoon on our honeymoon and it was one of the best things we did. We had two different guides, the first was Lance, who was hilarious and mightily entertaining. HIs knowledge was great, but his sense of humour is what made it even better. He had everyone (the English speakers anyway) in stitches with his constant jokes and puns while giving us information about the city and its landmarks.
The second guide, Richard, was also very good. Being a native Chicagoan, his knowledge was excellent, and you could see the passion he has for his job when doing these tours.
I'd highly recommend this to anyone visiting Chicago - money very well spent.