And it was better the second time through. If you didn't get in the secret door using the decoder then you are missing out on a large part of the game, both story and gameplay. After you fight your way through the secret lab you can choose two endings that you can't get from the wish granter. One of these choices lets you fight outside and around the sarcophagus which is very cool, once completed it's said to give you the "real" ending.

You can check out my review of S.T.A.L.K.E.R. here