Great song, very well done.
This song is, as many know, one of the best on Newgrounds. Now it also has some flaws, in my taste, but they are few and far between. Overall the song is brilliant.
The guitar riff at the opening is absolutely amazing. One of the best I've yet to hear. If this was used a bit more extensively in the song I would like it even more than I do now, unfortunately it doesn't make a return appearance for quite some time, or at least in a noticeable, stand-alone fashion.
Then the drums kick in and it sounds just about as good as the riff itself. Unfortunately, when the piano cuts in overtop the guitar riff gets covered up by the drums and piano, and it sounds kind of muddled. Improved clarity here would help the song a lot, in my opinion. Perhaps if you ever come back to and redo this song again, that could the area that needs improving.
Following that part of the song comes the piano solo, which is well done. Perhaps a tiny bit long, but pretty good nonetheless. Part of the drums disappear during this segment, which fits it rather well, as the piano isn't exactly D&B instrument.
Following this comes another very good segment of guitar solo, which has basically nothing wrong with it. After that we run into that somewhat messy meshing of the two like in the second part of the song. This also needs some clearing up by a tiny bit, make the instruments a bit more unique, and a tiny bit less clashing, as I've said before.
Finally, at 3:50, we hear the intro riff playing out again. It's still as good as it was, I'd say keep it.
So there's my two cents on where the song still needs improvement. It's bloody good already, to be sure, which is why I gave you a 10 and a 5, review and rating respectively, but there still stands some improvement to be made, like in all things. Keep up the good work!