I don't understand why people keep thinking that Cassandra is only eventually consistent.
It can be set to any consistency level you like for both reads and writes: http://www.datastax.com/docs/1.1/dml/data_consistency
It can be set to any consistency level you like for both reads and writes: http://www.datastax.com/docs/1.1/dml/data_consistency
EDIT: Just saying, failures and partitions are extremely difficult to deal with without atomic operations.
When we have R+W>N, and assuming N is 3, there is either a hit on read and write (W=R=2) or heavy hit on write (W=3, R=1) or heavy hit on read with a chance of temporary outages (W=1, R=3).
In HBase you write to exactly one server and read from exactly one server, which also provides for cheap atomic operations.