MongoDB is totally unsuitable for anything that requires ACID—but for things that don’t, it is unbelievably awesome. It’s even me that excited “kid in a candy store” feeling which (being a jaded software engineer) I haven’t experienced in a long time. It’s amazingly fast and seems to be smartly written, providing the perfect balance between schemaless document storage and powerful querying capabilities.
I can’t give enough kudos to its rockstar developers for being incredibly responsive to user feedback, too. I reported a bug about an assertion failure I was experiencing, and in 2 days, it was fixed. Two days! By way of comparison, I’ve been waiting upwards of two years for some bugs in Zend Framework that cause database failures to be patched (despite providing patches).
MongoDB even has an automated nightly build system, so it’s stupidly easy to test out the latest features and fixes. I’m certainly interested to see what it can get up to in production (especially its auto-sharding, since distributed clusters have typically been a big headache for me in MySQL) but so far during testing it’s been sheer joy to work with.