Cloudant seems interesting, never heard of them before. The pricing is intriguing. (https://cloudant.com/pricing/) From what I understand, it's free for most small projects but could get very pricey very fast, but then, handling millions of customers is far from being trivial to handle so I'm sure it's worth the price. Thoughts about the Cloudant pricing versus in-house solution (or vps)?
I might also note, thats its worth contact Cloudant directly if you are working on a larger scale production app. We have the metered pricing listed on the website and then dedicated which is an "all you can eat" style service and you pay a flat monthly rate based on how many nodes are in your cluster. That is the system that Postmark is using along with most of our other bigger customers.
