For what it's worth, when you read Vonnegut, you're getting the real Vonnegut. He put his heart into his writings, leaving all of us (and future generations) the richer for it.
I always felt like Kurt Vonnegut and Mark Twain were cut from the same cloth, but my impression was that Vonnegut was a bit too cynical (perhaps for perfectly understandable reasons) and his writing was less cynical than his person, whereas Twain came across as more WYSIWYG. It's nice to hear that his 'quirky, avuncular' side was very present in-person.
It's a very Vonnegut book, and a great gateway to the rest of his writing.
http://www.goodreads.com/quotes/279125-it-was-a-movie-about-...
His worst are probably Slapstick and Timequake, and the latter's so full of references to his earlier works that you wouldn't want to make it an early read regardless.
The short fiction/non-fic collections published during his lifetime are generally great. Big exception for A Man Without a Country which was pretty mediocre overall. Anything posthumous is suspect and should be deferred until you've worked through the earlier stuff, if not avoided entirely. Not that there's nothing good in there, it's just that the average quality is way lower.
Personal favorites of mine that don't generally make best-of lists are Deadeye Dick and Bluebeard. If you're reading him for the sci-fi connection rather than as general fiction, you want the Big Two plus Sirens of Titan (which would probably be #4 in a "big four" as far as importance-to-have-read) mainly.
I'd personally start with Cat's Cradle over Slaughterhouse Five. Deadeye Dick is strange and hilarious, but probably better after reading a few of his books first. :)
Probably true about Bluebeard, too. Both read almost as a review of the major themes and messages of Vonnegut's work, though Bluebeard more so than Deadeye Dick.