This lawyer disagrees: https://news.ycombinator.com/item?id=8104407

He even points out that using it could get you sued due to the lack of a warranty disclaimer by default.

I've heard other stories of companies refusing to use WTFPL-licensed code, and that alone should discourage you from using it if you goal is really to let anyone use the code with no hassle.

Edit: the FSF doesn't recommend it (in contrast to CC0, which they do recommend if you want a public-domain license): http://www.gnu.org/licenses/license-list.html#WTFPL