Well, I've messed around with it a little bit; it's pretty nice. The only current downfall right now (for constraction builders, anyway) is I don't think they have all the pieces yet; however, it may have been updated since I last looked at it.
Because it's free I would recommend downloading it and messing around a bit to see if you like it. Also if you are looking to bricklink pieces for a creation it would be super helpful.
You could use LDD, but they don't have all the bionicle masks and new addons; they have most other constraction parts though.