The key change - gimmick or legitimate musical tool?

Key changes have a bad name. This may or may not be largely due to Westlife and their stool routine. Sligo's most famous musical exports (and many others) use them towards the end of songs to bring the listener higher and higher as the song reackes its climax – kind of like this video demonstrates.
Michael Jackson is responsible for one of the most famous ones in Man In The Mirror (watch from 2:50 - and check out the word on which the key change happens) – and also might possibly hold the record for the most in one song (three!) in Will You Be There (listen from 1:32 on).

They can be done in really clever ways sometimes though – not always rising by a semitone or tone, and not always at the end of the song either. Below are two of my favourites.
Piano players – check out this beautiful playing on Nat King Cole’s Let There Be Love.
The intro is in A but there is a lovely change through D7 and G7 chords at 0:30 to get us to C by the time the verse starts.
Again starting in A, Brian Wilson surprises us all six seconds in to Wouldn't It Be Nice with a drum hit and a subtle C chord to get us to F by the time the verse starts.

The Beatles used them regularly, as well as countless other artists - and when used well they can make a song that bit more interesting - and hard-hitting!