Digital Moses posted an interesting article about the differences in the various engines' search results. Though this is something everyone is aware of to some degree, the actual study conducted by two universities shows just how different the results are.
A white paper of these results gives great details and interesting insights.
Add that to the fact that findings show the average searcher is not too loyal and the potential for swings to any engine seem larger than many may think.
So if a new engine emerges it always has the chance of taking over the space. Well that I think is a bit of a stretch but an amusing thought!