Seconded; we've had a very positive experience with it so far. We're running around 1.5TB in it (and growing) and it's working pretty decently.
Our biggest problems with it have been CPU saturation due to compression (solvable with sharding) and oplog size (due to supporting ACID; supposedly much better in the upcoming release), but both of those are surmountable. In exchange we get massively better disk usage characteristics, no global locks, ACID compliance, transactions, and generally better performance. It's not perfect, but it solved a lot of our problems.
Our biggest problems with it have been CPU saturation due to compression (solvable with sharding) and oplog size (due to supporting ACID; supposedly much better in the upcoming release), but both of those are surmountable. In exchange we get massively better disk usage characteristics, no global locks, ACID compliance, transactions, and generally better performance. It's not perfect, but it solved a lot of our problems.