It's nice. I used BrewTarget for a while, then switched over. I have to say that Beersmith is somewhat more polished and has a wider feature set, but in terms of producing and tweaking recipes, you can do everything you need with BrewTarget just as well.
Both have some strange interface decisions; I'm somewhat disappointed with Beersmith in this regard. I am tolerant of this in free software, but I expected a bit more out of something commercial. It's not terrible or unusable, just odd. The thing that bugs me most about both is the insistence on using equipment, mash, and fermentation profiles defined separately from the recipe. Certainly it's good discipline, but I very often want to play around with these settings and it's unwieldy. Beersmith is somewhat better in this regard, I think.
I'd say it's worthwhile having and knowing how to use *some* software. Beersmith seems to be the most widely used, which is a benefit, but if you don't plan on developing a lot of your own recipes and you're financially constrained, the free options work just or nearly just as well.