As with all video games, it depends on the gamer. I've played it for six hours and I absolutely love it. I've played guitar casually for six years and have played Rock Band and Guitar Hero on Expert mode. I love rhythm games and I love guitar. Rocksmith does a very good job of mixing the two.
If you have never played guitar before, Rocksmith introduces you to the basics and works with you at a very slow pace. If you have played guitar before, like myself, the game will quickly identify your abilities and adjust the difficulty accordingly. Therefore, your prior experience with the guitar is an irrelevant factor in purchasing this game.
A relevant factor is whether you enjoy playing guitar or think you will enjoy it. The game is essentially you sitting down and learning to play songs with a strong dynamic visual aid. This is awesome compared to the traditional method of sitting down and reading sheet music or tablature. It's also nice to get some feedback on how well your doing and where you are messing up.
Aside from learning to play the 50 songs in the library, the mini games are a lot of fun. So much fun that while playing them I wasn't even aware I was playing guitar. It just felt like i was using a new type of controller. I was surprised with how well the mechanics worked.
I haven't experienced any downsides of the game. I really think it's well worth the $80. But you have to like guitar. You have to want to invest hours learning songs. It's primarily a guitar teacher / trainer. But if you are looking for fun, the mini games come through.
Thanks for the input. Like you I've been playing for 10 years. I've got an SG Gibson just sitting and collecting dust. I've also got 3 kids which takes most of my time, so jamming with my friends is almost never. I figure this is a good way of creating my own jam session without taking too much of my time. My concern is (based on the reviews I've read) the dynamic difficulty; I've read that it doesn't read your skill level as well as it should and that it takes some hours of grinding before the game moves you into a more appropriate difficulty and challenge. Any truth to this? Thanks again for the input.
I personally haven't found the adjustment to be too terrible. You definitely have to play a song a few times before the difficulty it selects becomes meaningful (I've found around 3 or 4 does a decent job) but that hasn't really bothered me too much. I definitely haven't had to spend "hours grinding" to get the skill level right for my favorite in game songs (although I haven't been playing guitar for as long as you have--only about 2.5 years--so maybe the threshold it has to adjust to is a lot lower for me?).
I've also noticed that once I've "proven" to the game that I know a given chord by say getting a high ranking in a song that contains it, that chord will automatically appear in other songs that have it, rather than me having to start all over with the single note (ie. once the game has realized I know an open E chord, any time that chord is used in a song it appears as the actual chord rather than the root note or whatever single not it would have had me play before).
Hope this is helpful!