Doesn't add too much to RTS genre, but it's good enough to play through at least once
Gameplay is fun but gets a bit repetitive. Sure it's insanely fun when you have a bunch of spartans killing everyone or 2 scarabs owning your enemies, but those are it's 2 best moments. Those 2 little moments that every Halo fan wants to experience. It's always fun to upgrade everything and hit an enemy base with your army, but it's not something new to the genre. You do that in every other RTS. It's not a game that innovates, but it is fun nonetheless.
Storywise it does have its nice moments but it doesn't get any better than Halo 3, or MGS, or final fantasy games. Against other RTS games, its story is better.
Controls are pretty easy to adapt to, but you may find yourself pulling the right trigger trying to shoot enemies. Or at least that's what happened to me.
Achievements are pretty time consuming and you probably won't even try to get them all, unless you're a completionist or you like the game enough.
It's fun while it lasts, but doesn't hold against other RTS games.