This was the first game to utilise the idea of portals to "other places not necessarily in the same area, plane of perspective, or even on the same scale". I've been gaming since the bbc micro and I've seen my fair share of so called "gaming innovations" but this one truly had it as it came out years before portal, and gave me one of only a very few "no way, just.... no way *scratches head*" moments I've had in my gaming history in the events surrounding what looks like a moon in a box.... (portal players won't be blown away, but anyone else should be nicely confused).
This game also makes great use of the fact that there are more than two dimensions, so finding yourself fighting bad guys on the ceiling..or am I on the ceiling and they are on the floor .... or is it this wall..or...*I give up*..
This game messes with your head a bit and more, you also get shuttles to fly, gravity wells to muck about with and all manner of other very nice game enhancing goodies.
To summarise; if you've played portal it's like that but with guns n stuff, if not; still buy it, it's fun, looks great, endboss isn't about having the biggest gun (think strategy) and has a nice second element to it in the spirit world.
Do your face a favor, buy this game and give it a smile.