Bowie's box and the Station to Station release require you to fork out for vinyl as well if you just want the 5.1 mix. Pink Floyd have done similar thinks with their immersion sets.
5.1 sound is worth hearing if you have a home cinema set up but you should have to buy the vinyl version as well to hear things in 5.1.
Kink Crimson , Black Sabbath and The Talking Heads have released reasonably priced 5.1 mixes of classic albums without requiring fans to break the bank. So until the 5.1 mixes are available as a reasonably priced option my money will be staying in my pocket. I have the King Crimson , Black Sabbath and Talking Heads albums so I have plenty to listen to.