Ok, here are my impressions after a few playthroughs on expert for -almost- each instrument and song (Note: I am
very biased towards NIN):
March of the Pigs
Guitar - I don't know what to tell you for the charting of guitar for this song. You almost have to see it for yourself. Hard to get used to, holds followed by quick triple-strum chords and sort of a lousy pattern. Maybe I just suck, but I got 77% on expert. You get to play the piano part on guitar, though, so that fact makes it all worth it.
Bass - Probably the craziest bass song in the game. A LOT of energy (and, on my part, a lot of notes missed). The bass follows the beat of the song to a T and has crazy sweeps and hammer-ons to boot. Probably my new favorite Rock Band song. Prepare to strum fast!
Vocals - A very challenging song, to say the least (they
did put it in the Blistering Songs category btw). The hardest of the three by far and probably one of the harder ones in the game..just a lot less shorter. And, of course, no inclusion of the word

in this song - just a gap in the line
"I want to ____ it up".
The Perfect Drug***
Guitar - Awesome guitar in this song. You get a major part of the drum solo (that's where the chart gets kind of funky). Unlike bass, you are playing non-stop through the song. Not too difficult, I 95% it on first try.
Bass - You get to play bass maybe 1 minute and a half throughout the entire song, which is sort of a let down - but not unexpected. Pretty easy, straight forward charting with no real surprises.
Vocals - Easiest of the three songs to sing. Not too much pitch change - pretty straight forward. Medium difficulty song, tops.
The Collector
Guitar - Pretty moderate charting for this song. They use the guitar to do most of the piano, keyboard, and "extra sounds" accompanied with most industrial music. Like
The Perfect Drug, not too hard, not too easy (mainly the piano part).
Bass - Pretty straight forward song. I 100% on Expert the first try, and it seems to be a good choice for those looking for the
Flawless Groove achievement (or just a solid bass song).
Vocals - Awesome song to sing. Be sure you can belt out the screams on this one. A lot of pitch changes, and a fairly difficult song.
***= for all you drummers out there, try passing this song on Expert

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Hope this helps anyone who's on the fence!