Rocksmith is a music game, in the same genre as Rock Band, but it's also a guitar tutor. You plug your electric guitar, or bass, directly into the console with a special cable that comes in the box and you play along with the music, playing the notes that come down a highway towards you. You work your way up through various "events" and the game organises your playlists in order of difficulty. Do well enough & you get an encore track, do very well and you get 2! And you score points, which then lead to levelling up and unlocking stuff like new venues or new tracks (tho I think most are already available from the start). So that's the game side of it.
The tutor side of it is that it's actually teaching you how to play the instrument. As well as being able to rehearse or play songs straight through there is also a tutorial mode with a selection of "technique challenges" which each have a little intro video telling you what you're supposed to be doing and what you're supposed to be learning, and then a shortish piece of music to play along with that that involves that technique. The three I've played so far (for bass) have been about basic plucking, using two fingers alternately and playing syncopated rhythms. There's another couple of dozen I haven't even looked at.
There's also the riff repeater mode, which breaks a song down into its constituent parts. You then play that section over & over till you get it right, so next time when you play the song hopefully you can get it right there too. You've got a choice of modes within that - the one I've played most often is "leveller". In that you start off at a low difficulty level (which is also the case when you first play a song), maybe there's a note every bar or so. Then if you get it right it increases the difficulty, and repeat until you're playing the actual phrase as played on record. There's also "free time", where you start out at the most complex but it waits at every note until you've fretted it right and played it right - you succeed when you play through with no pauses. And there's "accelerator" where you start slow & work up to full speed.
And I discovered that if you manage to score well (ie hit most notes) when you've got 100% mastery of every phrase in a song it opens up a new mode to play the song in - Master Mode. Where it doesn't show you the notes, you have to play it from memory. Frankly that's a bit scary and I haven't tried it yet ;)
Obviously you only get out of it what you put into it - it's not going to make anyone a super guitar player in a week or anything like that. I do wonder if it's maybe a little too forgiving, I'm sure in the stuff I've played so far I've messed stuff up but had it register as OK. That might be because I'm in the early stages of the game side of it - perhaps the bigger (higher level) venues are more demanding, we'll see. It's also not a replacement for a teacher if you're serious about playing properly - it has no idea how you're plucking the strings for instance - so clearly you can pick up bad habits without realising it (or due to gaming the way it registers notes, I guess).
I don't know if I can judge how well it would work if you were coming at the game "fresh" - in addition to having had lessons on a variety of woodwind at school I've also played around on our bass off & on over the last decade & a half, so I've got a fair idea of what I'm trying to do even if I can't do it (if that makes sense). J says he finds the game a little more overwhelming than I do (too many notes coming at him too fast). He's still got a musical background but he's played on the bass less than I have, which may mean that if you're really new to music playing and/or to the guitar/bass then you'll find it all a bit too much. But then it does ramp up and down the difficulty as you play depending on how well you do, so perhaps you just wouldn't get to the "too many notes" stage until you were ready enough for it.
The obvious comparison is with pro-mode in Rock Band 3. I think Rocksmith might come out slightly ahead as a means of teaching the instrument based on my playing so far. Playing on an actual instrument means you're more likely to be able to transfer the skills learnt from the game to reality (given you're pretty much there already), combined with the Master Mode it means you can end up actually able to play the song without the cues of the game. And you don't need to buy a special peripheral or guitar for Rocksmith, which makes it a much cheaper option (if you already have a guitar or bass, but you're probably not interested in the game if you don't already own or are soon to own an instrument). But Rock Band 3 pro-mode has the advantage that the peripheral or special guitar has technology to detect where your fingers are, so the feedback for finger positioning is more instant.
(Oh, and the bass didn't need much fixing - it just needed the socket replaced coz the connection has always been a little dodgy when plugging it into an amp. And I got it re-strung at the same time coz I'm pretty sure it still had the same strings on it as were on it when we bought it in 1998. The local music shop (Jack White Music Store) did it in an hour on Saturday afternoon.)