I really liked the transitions, specifically the one around 50 seconds in.
My main critique is simply that the main portions of the song sound kind of empty/dull; the lead guitar is very prominent, but otherwise there isn't much "meat" to the song. The piano helps a little, but it still leaves most of the medium and high frequencies exclusively to the guitar. It might help to add some kind of supporting rhythm there, probably with another guitar track with more of a grunge to it, to give the song more of a sense of movement. I suppose the superficiality might be part of the genre you're going for, Drum N' Bass, but everything is drowned out by the guitar, the sound of which I find to be repetitive and a little obnoxious. Otherwise, the atmosphere is spot on and it certainly gets the job done in that regard. Feels like some big guy is about to beat the crap out of me. Good work.