I bought RS1 last October, switched over to 2014 'demo' after it was released, now I have both. Unfortunately, I have a fuckton of cDLC (player-made songs, requires a modified .dll to access debug mode on startup, look for them
HERE).
Favorites... I used to really love Peace of Mind, some of the unlockables are pretty cool too. And Judas Priest. Damn are those things fun to play. I also feel obligated to bring up Rush because
THEY ARE RUSH, GODDAMMIT, but the songs tend to be really technical.
As far as a tool for learning... well, I'm playing guitar for six years now, and the couple months after I got Rocksmith I found more improvements than over the past three years. Although part of it was also looking up theory on the web. It's a massive leap forward too, but it's easy to dive in waaaaay too deep and just confuse yourself horribly.
The Lessons is a godsend, it basically condenses every trick you need to know and then some, though it's slightly lacking at teaching you things like holding your pick right - it's basically described in general, plus 'whatever feels good', and this might lead to some problems that only surface at tougher parts (whether speed, accuracy or whatever), but hey, there's internet for that.
Also yeah, it lacks theory, and you WILL want to learn that at some point, but for training the more physical skills involved, it's great.