Well, NoSQL is a buzzword right now so I\'ve been looking into it. I\'m yet to get my head around ColumnFamilies and SuperColumns, etc... But I have been looking at how the data
RDBMS' are all about consistency. They do a great job on data that gets churned alot with transactions. See also ACID (atomicity, consistency, isolation, durability). Sometimes you don't need all that, like when storing data from logs or working on data that's not going to change, just accumulate.
NoSQL databases let you relax the requirements for transactions and get better performance (as well as scale to large distributed storage silos easier).