Why's it so good?
First of all, it's packed with Fischer's unique sense of humor. Read the first couple sample pages; if you're not laughing, you probably won't enjoy the rest of the book. The humor is black, definitely. But there's a good chance you'll be laughing HARD nonetheless. Pranks, absurd situations, physical comedy, and wicked wordplay rule the roost.
Second of all, it's dead serious. The book is about communism and the attempted revolution in Hungary in 1956. If you want to see the absurdity and insanity of the communist system as it looked from the inside at that time, Fischer delivers. It is fascinating, shocking, and it would be unbelievable if the author didn't make it so very believable.
I haven't seen anyone mention it, but Under the Frog reads a lot like Kurt Vonnegut's best work (Slaughterhouse V or Cat's Cradle). For me, though, Fischer's book has a lot more reread value -- neither the humor nor the horror has grown thin over the many times I've read it. Highest recommendation.