Very unique and a fantastic improvement over the original.
What kind of held DJ Hero 1 back was forcing guitar into the game thus limiting the soundtrack and the lack of freedom. DJ Hero 2 does away with pushing the idea that people who play with guitar controllers need a role in this game. There's no guitar functionality but you can have someone on vocals with two DJs mixing the track back and forth. This is an interesting set-up but the idea of singing/rapping vocals on a mix which is being scratched, distorted, rewound, and switching from two records, isn't too fluid or good of an idea. To get the most out of DJing you will really mess with the sounds of the song which makes it harder for the person doing vocals. It's just a nightmare to even attempt vocals in this game at times. Though anytime you have two people mixing the song it's great fun, the overall three-player experience is more competitive between the two DJs with someone singing on the side, rather than being a cooperative experience like say, Rock Band.
One thing that really makes this game stand on it's own is the freestyling features. Lots of segments allow you to do what you want out of many options and make the track your own. The game gives you some advice on maintaining and changing the rhythm properly and grades your freestyling right along with the scratching, crossfading, etc. This becomes an integral part of battling people as you can't just be accurate, you'll have to really have a bit of creativity when the game gives you chances in the freestyle sections. The battle modes are varied and have lots to pick from too, which is nice.
Empire mode in DJ Hero 2 is basically your career mode. You select a DJ (or use your avatar, if you dare), and pick from a list of clubs which you want to be your domain to bring the party to. From there you progress doing setlists and battling other AI-controlled DJs not unlike most music games. There's a few real-life people to take note of (Deadmau5e, RZA, David Guetta) and you can unlock DJs, bonus mixes, and even decks to influence your play with bonuses for certain different gameplay elements such as Euphoria (Also read: Star Power/Overdrive) lasting longer or bonus points on taps, scratching, or crossfading.
The only real issue with DJ Hero 2 is difficulty. It's a fairly well-progressing learning curve on the medium difficulty but as you progress higher, it gets messy. The hard difficulties in this game ask things of you that seem borderline impossible with the turntable controller. It's not a big problem but if you play rhythm games with the goal of mastering it, DJ Hero 2 is certainly one of, if not the most difficult to call yourself a master of.
The soundtrack is highly improved with some fantastic mixes, and gameplay is refined and polished in very superior fashion. DJ Hero 2 is a very fresh and high-quality music game and is among the best of them.