Long Way Round was actually one of the second hand books I bought for my trip to Africa and then promptly left on my bed. I had enjoyed the TV series when I had caught it on OLN and figured the book's always better, right?
My biggest problem with the book is the fact that I had seen the TV show... twice... so there were no real surprises left for me while I read it. I knew the people they were going to meet who would impact them; I knew the big arguments they would have; I knew the outcome of every crash. While I enjoyed reading it, it didn't have the excitement to it that reading Long Way Down had. I read Long Way Down (their second journey from John o' Groat's, Scotland to Cape Town, South Africa) before I saw the TV series and I really think that made all the difference.
One reason I did really enjoy the book was that you got a better sense of what they were thinking along the way. Although they do personal video diaries, often those can be somewhat jumbled as they try to work out everything in their memory from that day, whereas the book was written after the fact, using journals and memory so it's much more thorough. I also liked that Claudio, their always hungry, motorcycle-riding-and-crashing Swiss cameraman, became much more of a figure in the book than the silent partner you rarely see in the mini-series.
Both Long Way Round and Long Way Down offer a great chance to do some armchair travel, but I'd recommend reading them before seeing the mini-series. Get ready to fight the desire to get a motorcycle license until after you've finished reading.