I love it, and it is, as Zero Punctuation put it, a creativity toy with monsters. I'm currently building a castle around my tower which leads to my glass sky-fortress. I'm working on an external water elevator, and I plan to install a lava moat in the future.
After that I'm going to make a volcanic Island by building a floating platform over the ocean, then pouring lava down it. When the lava hits the water, it will turn to stone. I can also use that method to pour water down the sides of my fortress wall, then pour lava on the water to add width.
Then after that I'm going to dig a giant iron mine, then use the iron to build railroad tracks, and make a transcontinental railway system so I can more easily move materials from my various quarries and mines to my construction sites.
Really, it come down to if you like making things just for fun, if you do, it's the perfect tool for satisfying your creative urges, if not, then it's just a gigantic wast of time.
Really though, Zero Punctuation did the best review of it, you should look it up.