Having been live with NATS for 4 months and having used it for almost 8 months, I can say it's incredible in terms of the tools it offers, its flexibility, and the rich feature set.

The one place where TMM (makers of NATS) really falls short is on support. It can be slow at times, and their after-hours coverage even when things are disastrously broken can be really spotty. I've heard this from a number of other NATS users and experienced it myself. And TMM has a reputation for being exceptionally arrogant, and the reputation is not undeserved. If their product was fabulous, perfect, and bug-free, then they might be entitled to their arrogance, but they are still a young company with a not-quite-mature product.

If we had it to do over, I think we'd still choose NATS over MPA, but I do think that MPA deserves a close look. What I've heard from people who use both is that NATS is more flexible and feature-rich, but MPA has much better, friendlier support, and the product is a little less buggy.