Being a geek, there is a certain canon of literature you are expected to have read. Hitchhiker's Guide is one of the books on that shelf. I saw the movie some time ago and really enjoyed it. It was wacky and fun, and didn't take itself seriously. I bought the collected works of the series so I could fully immerse myself, not just dabble. After finishing the collection reading straight through, I think this was a mistake.
It's not that the books aren't enjoyable, because they are. It's that the style is sledgehammering consistent throughout. The wackiness of the storyline just starts to wear. I think I would have enjoyed the collection much more if I had simply read one book, then carried on about my life for some time before trying the next. As it stands, I don't think I fully appreciated each component of the collection. The first book "The Hitchhiker's Guide to the Universe" I enjoyed very much. Life, the Universe, and Everything kept me turning pages as well.
I think part of the problem is that there is simply no plot. The stories follow the life of one Arther Dent. A wholly unremarkable fellow who is forced through an incredibly remarkable life. Reading over 800 pages in succession without any form of plot to pull you along can get a bit tedious. Which is why I heartily recommend that if you want to truly enjoy this series, read one book, then go away and do something else for a while. Do read it, though, as Douglas Adams use of the English language and storytelling are very enjoyable. As to this edition of the book (The Ultimate Hitchhiker's Guide - Deluxe Edition), I can only recommend it in so much as it will be mistaken for a Bible when you are reading it on a plane, and people will treat you nicely for it.