It's very odd for anyone who can really notice what is going on. Myself, I don't play guitar and play bass guitar VERY POORLY, but it still really annoys me. It's not that you won't be able to play the songs, but you will definitely do double takes if you are trying to follow the rhythm. It gets a lot better (but not perfect) on Hard and Expert.
(I am teaching myself bass)
Easy and medium seem more like if you are learning a song that you just play chords/root note to get the pattern.
I don't have issues with easy or medium. However, you can't look at it like playing a guitar, even though the controller looks like a guitar. It's DDR for your fingers.
oh of course you can't look at it as actually playing a guitar. I didn't mean to imply that. And my statement about Easy and Medium isn't that I can't play them... just that I actually find it frustrating at points when you can clearly hear notes yet you are not playing them. The speed and stream of notes on Easy and Medium make it very simple to do well on, but if you really listen to the song as opposed to just watching the little note bumps whiz by, it's a different experience.
In certain songs, you are definitely just playing root notes, in others it switches to completely different instrument tracks. It varies by song, with some working well and others just being completely weird IMO.
You won't learn to play guitar with this game, that's for sure

It's a ton of fun though.