MongoDB has single server durability since 1.8 with the journal. If you put the journal on an SSD, you can even get it almost for free performance-wise.
Yeah, I guess I should make the point that if you don't put the journal on SSD (and you only need a few gigs of SSD to journal terabytes of spinning disk storage), you will see significant slowdown.
You could say that about any database, though - nothing special about mongo's journaling that makes it "for free" wit h an SSD.