Invasion seems nearly impossible to win on defense. I always end up on D and we never hold ground long. This mode has a lot of potential though.

cant say i've had this problem.
Always felt 50/50 in the game si played, sometimes we won, sometimes we lost

i must disagree with you, i always win as spartans and very rarely as elites, ive played with a group as elites and we were all comunicating but the spartans just had better weapons and often killed us before we even got near the base

Invasion is ok but i feel like the elites do have a little bit more of a stronghold. I won 3 times as an elite it is fun. But 3 games 3 times elite in a row it gets annoying after awhile...

I've heard it both ways. From what I can tell, it seems that the team to organize and cooperate wins. This may bother all you loners, but I for one am glad they have a new team based system. It's not based on twitch skills anymore folks, it's tactics.

i think whoever has the best teamwork always wins.
it is rather difficult as an elite sometimes, but if you can get 3 guys on the side of the spartans you can usually spawn the rest of your squad and then attack all at once.
Usually regular invasion is more spartan lopsided, whereas invasion slayer is tilted towards the elites.
Though i think the spartan weapons sort of outmatch the elites, yet the elites are tougher and can evade.
I think it's typically fairly balanced though, maybe a slight tweak could even the odds just a touch though i'm not sure what.

"The team to organize and cooperate wins"

Well put. I don't get why they divide each team into groups of two.