I'm too cheap to buy army builder, but I have borrowed it on occasion. Its really not faster than just writing it out.
The thing is, I have the points cost for most every (empire) unit memorized, and what's more, since my army lists are usually only different only on a couple of points, I have the exact value of my more constant units memorized as well. Army builder is a help when you try
to create an army to look neat and organized, and it beats doing pencil and paper/ calculator math, but why bother buying it when I can pump out a list in 5 minutes anyway?
One of the reasons I might buy army builder though: You could create your own list for an army whose book you don't have! Someone I know does this a lot, and then brags because he says his army would crush me, but then I tell him to buy the army and then we'll see...