Hey JAZZA!
This is definitely one of the better covers here on Newgrounds, no question about that! The piano part was played well and I think it had just the right presence in the mix; loud enough to be heard, but still in the background. The piano sounded very well recorded as well, props there!
Although there are a bunch of great things done here, I've got two suggestions for you. For one, I think your vocals would clean up pretty well if you run them through a pitch correcter. The recording quality is great, and most of the time your in the ballpark with the pitches, but there are a few notes sprinkled into the mix that are a bit off. Sonar has a built in pitch correcter that works quite well, and there are a few free VST plug-ins around the web that do their job. Aside from that, my only criticism is in the drum tracks, which sounded quite synthetic. I think bringing out the mids in the snare and the lows a bit in the kick will help give them a bit more life, but ultimately it may be a good idea to invest in some real quality drum samples.
In perspective, the pros far outweight the cons, and this is done very nicely. Keep up the great work!