I do. I used it for the 21 days of the trial period and when the trial ran out I wanted to play around with designing some beers. Without beersmith I was totally lost. It's totally possible to do the math on paper using the basic formulas you can find in How to Brew but come on, it's 2013. I'm not using pencil and paper to do math when I have this laptop.
My only regret was that I didn't know one of the vendors here on HBT has it for a discounted price. I could have saved 7 or 8 bucks.