unlike the PC version, you DONT need C&C3 to play this game. it works seperately.
the game itself is really good. in terms of the armies, there are now 2 additional sub-factions for each of the main sides (GDI; Nod; and Scrin) making it 9 armies to pick from.
sadly the story campaign is pretty short however, which is the main criticism and it only involves the Nod factions meaning that it restricts the purpose of the extra sub-factions which is a shame. at least you can use them in the skirmishes and in online games though!
the control system has been greatly improved since C&C3. its pretty similar, but the vital addition of the 'Command Stick Interface' makes life much easier (you hold Left Trigger and use the analouge stick to pick an action/menu represented on a 'circle'). as a result, ive found this game really enjoyable and less frustrating to play than C&C3 (as much as i really liked that as well, the controls was a real issue).
the exclusive Kanes Challenge mode is a nice addition. it makes up for the fact the GDI and Scrin armies have no storymode, but is repetitive as all 9 factions have exactly the same missions to play through.
overall, its excellent. the short story mode is the main criticism, but the new control interface is excellent, and its great to see it carry over to Red Alert 3. C&C fans should definately buy this.