I tried BeerSmith a few years ago but it was before he made a lot of the improvements around mobile and web syncing.
In the meantime I found Brewer's Friend and it has worked well for me. The thing I like about it is that it's web-based, so I can use my PC at work, my MacBook at home or the iPhone app and keep it all synced up. I think BeerSmith now has similar capabilities, but someone that uses it would need to weigh in.
As Lee states, regardless of which tool you select, it really just creates a framework in which to map and track your process. Spend the time to measure out exactly how much wort is left in your kettle and get that into your equipment profile. Same with temperature drop in mash strike water, trub left in the fermentor, etc.
The software just helps you dial all of these things in and make the process repeatable. Once you get to that point, it also makes it easier to scale and share recipes.
Also in agreement with Lee on Bru'n Water. It's a great water tool. Brewer's Friend has a water calculator as well, but I ended up using both at times.