Yes, it's very useful. I fire it up every so often (every month or two) and let it search for unused items. It can be over-enthusiastic so I tend to ignore much of what it suggests - like deleting unused language files from apps and removing iTunes artwork caches. That said, it provides a curated list for you to choose from including log files, broken login items, broken preferences, etc. Since I do software development I can end up with massive log files that I simply don't notice. I just recovered 10GB of space due to that.