We approached Stripe with several million/mo in processing across hundreds of merchants. The rate they offered us was almost as laughable as 2.9%+30c.
Their docs aren't as good as Stripe, but there is greater functionality available. Overall, I'm satisfied.
Interchange plus is definitely the pricing model that most businesses want to be on (and it's actually not limited to high volume businesses - plenty of small merchants can get it, too) but be aware that by itself, interchange plus isn't a silver bullet. It really needs to be a true pass-through model, where you pay the true costs of interchange and assessments. Otherwise, you could still get overcharged. Mercury Payments, for example, charges with "interchange plus" but has now been sued twice for overcharging on assessments. Basically they padded the costs of assessments while trying to make it look like those were the costs charged by the card brands.