It's a very easy game. Most problems are with a few misprintings on dials and the clear rods some of the hovering/flying models use can break kinda easily. Some people say the vehicles are too powerful, and some say the opposite. No idea myself.
My biggest problems are that at least half of the models you get will be spartans, and I understand why they did it, but it is annoying none-the-less. And because of copyright issues with bungie splitting from microsoft there might not be any more sets.
Great models though. I think they're all based on the CG models and almost all in scale. What a novelty.
For vehicles I'd suggest checking out the McFarlane Toys Halo vehicles, they're a bit out of scale one way or another, but in general they work pretty well.