I have never once heard a bad Rautavaara piece. It is excellent stuff. A great introduction into contemporary-style music because it has some fairly dissonant/abstract writing, but is by no means inaccessible to even the most under-exposed listener (though personally I first heard him after getting a fair taste of such music). But beyond just being a good "introduction" he is a superlative composer.
I can't really say much more. For information, google his unique name and you'll find whatever you need. For recordings, there are some great sets on Naxos. Try his "Angel of Light" Symphony, "Isle of Bliss", symphonies, and Cantus Arcticus for orchestral works. His piano works are phenomenal, but I recommend starting with his concerti. They are unbelievable...
After that, his etudes are excellent and, I imagine, quite difficult (haven't seen the scores though), and be sure to check out his sonatas, preludes, and icons. Fantastic!
He usually writes in a very "spacey" style, referring to actual outerspace if that makes sense. His works, at least to me, tackle questions of time and eternity and have a certain quality of "complete vagueness" about them. He uses some hauntingly hollow harmonies which to me evoke Arctic atmospheres infused with a strange evolutionary process embedded in the pieces throughout. It reminds me a lot of some Ligeti.
But that's rather subjective of me. I am no expert in his music, or of any music for that matter, but am really in love with the composer's output thus far. He is one of the best composers of our time, I think.
Topic: Rautavaara