I've been a fan of the Guitar Hero series since World Tour came out, and I decided to finally change from playing Guitar Hero only on the PS3. And so I saw Guitar Hero Modern Hits in HMV for £10, including the guitar grip, and bought it. For a Nintendo DS game, they managed to get quite a lot onto that one little cartridge. There are 28 playable songs (I think), which is pretty decent in my opinion.

Gameplay - the same as any other Guitar Hero game, except you have to drag the stylus across the touch screen in order to "strum" instead of having something like a strum bar. And there are only four fret buttons instead of the five that you get with guitar controllers for consoles.

Songs - a good selection here actually! I was pleasantly surprised to find a couple of my favourites in there, along with several songs that I've come to love since discovering them in the game! If you're a fan of the kind of songs on Guitar Hero: World Tour, then you'll probably find something you love on this game.
Examples: The Offspring - Half-Truism, Placebo - The Bitter End, Bullet For My Valentine - Scream Aim Fire, AFI - Miss Murder

You play through different venues, unlocking new ones after gathering a certain number of "fans". Playing a song for the first time also unlocks different challenges (under the name of "Fan Requests") for that song, including getting a certain score while playing a bass track, or using a certain number of battle items in a guitar duel, etc. This is actually my only problem with the game. Because you have to get so many "fans" to unlock new songs in career mode, you HAVE to play a lot of these challenges, meaning that you can play a song through two or three times before you can actually get different songs to play. There are three Fan Requests for each song, so you could actually end up playing each song four times if you wanted to, three times on lead guitar and once on bass.

Other than that, I would highly recommend this game to any fans of the Guitar Hero series. You can always just put the game down and pick it up later to play those same songs again, so that you don't get quite so sick of them! :P