Depends on the manual. If it's a thick tome that clearly the developer intended the player to read to have any clue in the world what the hell is going on in the game (Capitalism II and The Witcher, I'm looking directly at you) then I'll RTFM. If the manual is a "booklet" that has all the production values I could get at home with a printer, some A4 paper folded in half, and Microsoft Word, then I won't waste my time (ironic that the more time I have to put into it, the less likely the time is to be wasted.)