Take Morrowind, for example. Before that game came along, I was used to linear games. Maybe I can get away without the guide. After all, once you get so far, you're finished with the game anyway. Cue credits. I win. After playing it for 20 minutes, I knew how gigantic it could potentially be. When I saw the size of the guide, I knew I had to have it.
I expected Oblivion to be equally (and then some) overwhelming, so I reserved the guide again. Then I found out that everything is handed to you via your handy dandy compass, and all the locations are jammed into a small land mass anyway. Didn't really need the guide.
I also assumed FO3 would be overwhelming, and got the guide. I'm glad I did, because there's a lot to cover on that land. It was much more difficult to fully explore than Oblivion.
For TES V, I'm getting the damned guide. Again.