I distinctly remember this game being a BLAST to play. Tons of action, an intriguing plot, and Japanese anime influences abound. Even if anime isn't your thing, there's still PLENTY of run-and-gun action to whet your appetite for some FPS action.
The 'giant robot' mode is really just a modified FPS mode: there's still human player characters, but they (and the level around you, when you pilot the giant robot) are scaled down such that they appear tiny. So the people are still detailed, but the game otherwise plays the same. This is a GOOD thing; that means it's actually NOT very difficult to control your mech, but you still get that 'mecha piloting' feel because of good level design! Plus you get some really badass weapons.
My one really big gripe is that the AI seems to be a bit lacking, especially in the on-foot missions (as noted in other reviews here). While not a deal breaker for me, it can occasionally become a tad frustrating. But don't let this deter you: this game is TRULY one of the great "classics" of PC gaming and must not be missed!
Did I mention how much fun multiplayer is, too? Such a blast...