As a novice homebrewer (only brewing 2 batches thus far). I find BeerSmith to be a useful tool. I like the fact that I can look up most any grain, hop, yeast, or style of beer and get some information on it. I like seeing how the additions of certain grains can change the gravity while others barely budge it. I also enjoy the timer, being able to take notes, and create your own system setup that is exactly tuned to what you personally have.
Like others have said, It won't tell you if your beer will turn out good or not, but it will provide a snapshot of just about everything you can imagine, so when you go back and analyze your brew day, temps, what happened when, etc, you have it all in one neat, easy place.