I really enjoyed SoC and CS - the former for its scare factor; the latter for its narrative focus and better AI - but CoP did nothing for me, as they dialled back the graphics and they simply hadn't done anything to advance the formula. The genre had simply moved on and playing through a rehashed version of the previous two games was a real grind.
I'd love to see a proper sequel with a new engine and more refined gameplay, as it's certainly a compelling brand. It's a shame that they made such a mess of development - I imagine that running years behind schedule with the original game was probably responsible for most of their financial troubles, while the technical issues that plagued the series must have put off a lot of people.
Metro 2033 was a much more polished and narratively driven game, created from a splinter group that worked on the original STALKER. It's a complete different genre of game but it was much better than CoP. I'm really looking forward to Metro: Last Light.