Dynasty Warriors 7 is a tough one to rate really. For me, its a 5 star any day (huge fan of the series/developers for years), but for those not versed with the Warriors series it's probably much less so.
For those that aren't familiar, Dynasty Warriors 7 takes you back to the days of Ancient china, and follows the novel "Romance of the Three Kingdoms". It has a huge cast of playable characters (62), a ton of stages (and more via Downloadable content if that isn't enough) and I combat system which you will either love or hate.
Now, speaking of the combat system, it is pretty much a 'button masher'. There are two main attacks (Square and triangle) and these can be combined to make 5 or 6 different attack combos. You then have special 'musou' attacks which are both strong and visually impressive. There are a total of 35 different weapon types in the game, with each category having a further 10 or 11 weapons of varying strength.
A stage basically consists of cutting through hundreds/thousands of common enemies, with stronger, slightly smarter opponents in the form of officers. The game presents a fairly good challenge on higher difficulties, although most of the common enemies do stand around not really doing much, but this is something those familiar with the series will know.
For those familiar, the main things to note are that there are now 2 different weapon slots, so characters can switch between weapons mid-battle, which is a new feature for the series. This means that not all characters are individual anymore, although I personally like the new way of doing it. All the weapon types have fairly fun movesets anyway, and there are only a couple I didn't enjoy using. The story mode has also been overhauled, in that you control a specific character in each stage, and the story has a lot more cutscenes and dialogue. generally it is the greatest step forward the series has made.
So in conclusion, I'm going to contradict my original statement slightly. If you are a fan of the series, it is a must have as it improves on Dynasty Warriors 6 in every way, and if you are new to the series it is probably the best place to start.