DUW or Disk Usage for Windows 95/NT is one of my favorite apps, especially around spring cleaning, but indeed it does have a few issues.
I'm going to tell you, so you're not surprised when you run into them ;)
Firstly, when you install disk usage for windows, like usual it will choose a default directory to install to (which you can change). The only problem is that the directory it chooses by default is c:\duw, not exactly the best choices in directories. This can clutter your C:\ drive leading to madness ;) Seriously though, applications should install to C:\Program Files, but I'm going to let this one slip since its more of a cosmetic defect than anything else.
The second problem is a little more annoying and can affect you if you have massive directories. It simply will report directory sizes wrong if you have very very big directories (over 2 gigabytes). For example say you have a 4 GB drive and you have one directory that has home videos in it, and they add up to 3 GB. Well disk usage for windows won't report that directory size correctly, it will actually report a negative number! Now that's kind of funny ;p
All in all I have to say that these issues are minor, the first issue being minor, and the second because it doesn't affect me very much, only once in a while, and hey, its fast and free, so its still one of my favorite tiny apps. It deserves that moniker, its under 200 kb, that means its less than a fifth of one megabyte! Tiny indeed.