A fair point. My datasets have been relatively small, topping out at less than a gigabyte. I didn't have any crashes in production, and I had a small enough database that I could export the whole thing to the file system every six hours. Dumping and restoring from that dump was fast.
10gen has delivered, time and again. They're looking at ways to be smarter about replication, and I am confident that by the time I need that level of durability, they will have brought the product up to where I need it to be.
The additional productivity I feel I get from MongoDB is, of course, balanced by the knowledge that I am working on a platform that is a bit of a moving target. It's been a wild eight or nine months in some ways, but I've been thrilled with both the product and the relentless drive from 10gen to improve an already kick-ass tool.