Hello,
I started reading the book a few days ago after an atheist friend of mind recommended it to me. I am soooo mesmerised. The books has amazing characters (I love Marvin), and great philosophical arguments (for example by giving the number 42 as an answer, Adams settled the question of what is the meaning of the universe once and for all; it is a meaningless stupid question that can only give stupid answers).
I absolutely love it and look forward to reading the rest of the series.
I'm glad you're enjoying them! I've read the series a couple of times and really enjoyed it (I'm probably due for a re-read). In line with what @Hastur said, I found the first two books brilliant, but it started meandering after that. "Mostly Harmless" was more or less to sate die-hard fans who wanted it to go on. But please don't let that sway you - they're worth a read. Also, I think the series and the film are also worthy.