I'm a big geek for special features! I quite often find myself opting for 'Special Editions' over a bare bones releases. However, I'm not really overly fussed about extended editions of films - as in most cases, they tend to be extended by about 2 minutes.
The Alien Quadrilogy is leading the way in terms of special features. Two versions of each film and then literally hours and hours of behind the scenes footage and commentaries - such an amazing collection. I picked it up for £19.99 on Blu-ray and I think it's even cheaper now - so in my opinion, it's a must own title on Blu!
Other notable collectors editions have to be the extended editions of the LOTR trilogy - the feature length docs on each films 4 disc package is worth the price alone. And the films actually deserve the title of extended editions.
Memento has a cool 3-disc collectors edition available and features some cool content - including a rare commentary track with the often elusive Christopher Nolan.