I use lots of different software for recipe formulation. Not because i need to, just because i'm interested in how they all work.
I use Beersmith. Have used it for the last few years and enjoyed it, but then went to Brewmate which is also excellent and created by RandyRob - AHB member, and it's free! Although as BribieG mentions, i too would not hesitate to part with cash to use it if he decided to start charging for it.
Of late however, i have gone back to BeerTools Pro which is also excellent.
They all take a bit of getting used to, as they all operate in slightly different ways, but that doesn't worry me. It's part of the learning process to me.
Brewmate (i think is by far the easiest to use) and it's pretty bloody accurate i've found. It may be all you ever need.
Beersmith is massively complex if you want it to be but once you work it out, is also easy to navigate.
BeerTools Pro is by far the most "modern" looking of them, but it also works differently.
The other "main" contender is ProMash which is also excellent, i've just never used it.
Beersmith will be updated in a month or so, Brewmate keeps getting updated constantly (thanks Rob), and i believe i may have seen something about ProMash getting an upgrade soon too.
Try 'em all i reckon and see which one does what you want (they all will) the way you want it to do it.
Cheers,
Nath