BeerSmith was integral in getting me into all-grain. Like any tool, there is a learning curve, and you can't expect something as low-demand as homebrewing software to have an interface as polished as the latest release of iTunes or Firefox. Having said that, BeerSmith is packed with features, and really helps me keep my brewing universe organized.
I started out using BeerSmith primarily for building recipes--calculating the right grain bill for desired the desired starting gravity, figuring out IBUs from hop additions, beer color, etc. When I switched to all-grain, of course all those temperature and water volume calculations were important, too.
Now I find myself relying on more of the "secondary" functions in BeerSmith. Using BeerSmith to manage my inventory is a great time saver. When you have bulk ingredients on hand (pounds of hops, 100# plus of various grains, etc.) it's nice to be able to see how much you have of each ingredient from your PC rather than having to do physical inventory and keep everything straight as you punch up new recipes. When it's time to brew, you just click "remove items from inventory" and it subtracts all the stuff you are going to use so that your inventory list is still accurate. If you need to go get certain ingredients before brewing, you can click "print shopping list" and take your list to the store with you.
I'm also easing myself into using BeerSmith as my only brewing calendar. I'll admit I still write on my office wall calendar, and a lot of my brew day notes don't seem to make it onto the PC, but I'm hoping to get in the habit of putting everything in BeerSmith so it's all in one place and I don't need to shuffle papers anymore.