The 2nd best "thinking" FPS ever (behind System shock 2). The visuals are awesome and still work well. Action is good and the upgrade system, interesting guns (especially the disc), and even the story with some funny twists still keep things way more interesting than most modern shooters. Having the general power/memory of the device you are in reflect in the upgrade system is very nice touch. Still worth it.
This was the final strike that made me GoG user. Best PC game ever made. GoG version is MUCH easier to install than original (which I have as well). Just remember to get SS2Tools v 5.0, shtup beta 6 and rebirth v2. These really make a difference in game graphics. Also, this game is one of the best Co-op games as well (but play it once through alone first).