Personally, once I found Gamefaqs, there were only two reasons I ever bought strategy guides. The first was to get production art and elaborately detailed statistics for complex games like Disgaea and Makai Kingdom. (Doublejump Books is by far the best commercial strategy guide company on the market. Bradygames is okay, while Prima's guides are complete rubbish.) The second was for games like Valkyrie Profile, in which maps and visual aids are key to understanding how the heck to proceed at some points. If StrategyWiki lives up to its promise, the second reason to buy strategy guides will be gone.
Of course, Wikis have their own drawbacks, as Tycho of Penny Arcade so eloquently pointed out last month. Given how contentious debates over what the optimal strategy for a given game is can be, it will be interesting to see how StrategyWiki deals with the inevitable differences of opinion that will crop up.