IMO the first book in the GoF series is the best. The first 3 weren't bad over all as they all had the Carl / Karl character to carry them over but once he died it became a bit tired.

I still have the first 3 (somewhere I think) and I don't mind them. Mind you, I don't rave about them but I thought they were okay.

I read Dune a few years ago and really, really liked it, even with all the hype.

For terrible disappointments, check out (or, better yet, avoid like the plague): The coming of the King by Nikolai (yes, Nkolai) Tolstoy.  Great grandson of Leo, writing about Merlin.  Should have ben good, but was the only book in the last twenty years that I've actually laid down unfinished.  Other major disappointments have been the newer work by David and Leigh Eddings, especially "The Elder Gods".  I loved the Belgariad, the Malloreon and even the Sparhawk books, but everything since has been much worse.

Man,

 Bad books? How about bad movies like Starship Invasions? (circa 1977)  

Guardians of the Flame by Joel Rosenberg. I remembered reading this when I was in junior high, and the premise drew me in because I was as a kid a huge D&D geek. Roleplayers who suddenly find themselves in the fantasy world as their characters, their actual personalities morphed with their characters! What made it more intriguing was rather than a case of teen wish fulfillment, they found themselves in a world that was hard, harsh, unforgiving, and many of them quickly began to miss their cushy American lives. Some died--and when that happened, it wasn't merely an excuse to roll up a new character: they were dead. In fact, I think that as an adolescent I only got about halfway through the series and then lost interest because it was too dark.

So, I picked it up again about a month ago, curious whether I'd appreciate it as an adult. The fact that I remembered it as being dark was a promising sign: to do this idea justice, it would have to get pretty grim.

I only managed about four chapters this time. In Rosenberg's defense, this was his first novel, but the writing struck me as sloppy and amateurish, the dialogue clunky. Maybe his writing improved over time, but I don't have time--with so many other books calling to me--to give him the benefit of the doubt. So it looks like the series will, for me, remain unfinished. 

Little People, by Tom Holt. I'm a fan of his early work (Expecting Someone Taller is a comic fantasy classic) but I was unable to finish this one. There were simply too many plot holes and inconsistencies for me to swallow. If I'd been his editor I'd have sent it back covered in red ink...