I bought a 360 add-on shortly after the death of the format off of eBay when everyone was panicking and dumping their players dirt cheap. $30 got me the player (barely used in the box), American Gangster, Hot Rod, Dawn Of The Dead, The Kingdom, and King Kong. Since then, I've spent maybe $60 total to add Transformers, 300, Happy Feet, the Bourne trilogy, Planet Earth, Jarhead, Alpha Dog, Hot Fuzz, Big Lebowski, and Heroes S1 to that collection.
There are still things I'd like to pick up (namely the Harry Potter and Matrix sets), and if they're considerably cheaper than the Blu version, why not? Whenever I pick up a PS3, I'll start getting things on Blu but until then, a dirt cheap copy of an HD movie works for me.
When I bought Planet Earth, it was $20 as opposed to $65 for the Blu version. Easy decision.