I don't even think of it like a normal Micro-Transaction, I think of it like acquiring tickets I can redeem for DLC if I want to. It really doesn't effect the game at all, it may be the best implementation I've ever seen.

I honestly didn't even know there was a purchasable currency system until I'd played for a week or two, by then I already had 3/4 the amount I needed to unlock a new tier of gear. Each new tier costs 1000 credits, you can find 0-100 credits on each mission, depending on how much of the map you explore. Missions take 45 minutes maximum.

I shouldn't even be saying tiers, they are more like collections.