This is not a game, so it shouldn't be reviewed as a game. Its sole purpose is being a repository for these 21 country songs, the playability of the disc is just an extra perk. It's just a stripped down Rock Band 1, it has everything you need to play the songs and nothing more.
Since this isn't a game it should be judged primarily on the fun factor of the songs, and my verdict is that they're overall very solid. The pack is mostly contemporary country with 2 classics thrown in (Willie Nelson and Kenny Rogers). Sure, it has a couple simple songs but the drums and guitar for the majority of these songs is surprisingly intricate. Banjo is very fun to play, it's always all over the place and littered with HO/POs. Bass is 99% nothing special, and vocals are fun if you actually know and like these songs.
I'm not a country fan myself but I'm open minded and I genuinely enjoy 3/4 of the songs included here, some of them I've even grown to like as songs. If you're a country fan this disc is a great deal since you're getting 21 songs for the price of 15, and 13 of them aren't available to the general population yet.
On the merit of the songs alone I'll give this 4 stars.