How do you determine what sets you will buy? Price per piece - playability - theme - gut feeling.

I try to follow all four of these, but theme and playability are usually the highest on my list.

Solid market trends and history generally prevail in my book (ie. 'theme') - from an investing viewpoint. Playability doesn't really rate highly when I look at and evaluate a set - afterall, I doubt many AFOL's actually play with their set, other than to admire it and appreciate it as a part of their collection. I try to establish whether a set will 'display' well, contains 'hidden gems' (openings, places for minifigures, subtle yet important design details, etc), or offers a 'rarity' like a particular figure or part. The generally perfect example is the UCS Star Wars. It does all of the above with ease, and rarely fails. But, even the UCS is prone to weakeness. Take the SSD (10221) and look at the underside - a failure of note. How can such an iconic ship not display the same style of undertones as the ISD. Go figure?!?!