Having recently re-experienced the original Perfect Dark through the XBLA re-release, I decided to take a gamble on Zero since it was dirt cheap. I'm enjoying it but your review does make fair points, particularly about stealth being difficult thanks to insanely resilient and observant enemies. I've been pleasantly surprised by the multiplayer 'combat simulator' though. There's no one playing it, but even the bots are nicely challenging.
'There would be tears and there would be strange laughter. Fierce births and deaths beneath umbrageous ceilings. And dreams, and violence, and disenchantment.'