At the time of Modern Hits' release, it is the third version of On Tour in less than a year.
The game is, well, Guitar Hero. You play songs, progress further, play more songs. Modern Hits uses the same hardware as previous On Tour titles, the handheld four-button piece of equipment that you slot into your GBA slot in the DS and DS Lite (keep in mind, this means that it won't work on the DSi). Holding the DS sideways, you strum with your stylus while using the new hardware to press the buttons at the right time to pass songs and win fans.
Modern Hits changes the formula a bit, however. Instead of simply playing through a set of songs to progress, you must gain a certain number of fans, gained by playing through songs and a new feature called 'fan requests'. These requests mean fulfilling special requirements in songs or guitar duels, sometimes while playing different parts of the songs than you're used to, such as the bass.
The new way of doing things is a refreshing change, but it has problems. The biggest one is that each set now only has three songs in each, which means you'll be playing those three songs, and variations of them, until you've managed to unlock the next set. If you're playing songs you like, great, but if not, you've got some unpleasant times ahead. The songlist itself is, as always, a mixed bag. It's always a subjective subject, but chances are you're going to like a bunch of the songs and dislike others.
It's hard to recommend Modern Hits to anyone who has either played the other On Tour titles, it's just so similar to the previous titles. If you're a big fan of the games, then it's likely you're planning to buy this game anyway, as the fact that the game has the same engine means that you're going to get the same experience from it, good or bad. With the release of the DSi, this may in fact be the last On Tour title, so if you want to experience a portable Guitar Hero experience, this may be your last chance to.