S.T.A.L.K.E.R. is deeply divisive, and not just along the love it/hate it line. Both games are a combination of brilliant concept, unrealized potential, and shaky execution. People either love the concept enough to overlook the manifest flaws, are disappointed by what's clearly a good start, or just outright frustrated by its quirky gameflow.
For those of you that like it, but are frustrated by the unrealized potential (raises hand), the good folks at GSC Gameworld have released a pre-alpha from the "Oblivion Lost" days (2004). I'll post my reactions when I get 'er downloaded.
Incidentally, if you own Shadow of Chernobyl, check out the Oblivion Lost mod. It re-implements a lot of disabled features that are still in the code (all Hot Coffee style), which results in a much richer experience, even if it's distinctly rougher around the edges. Recommended!