Edit, Nov 2017: Please note the original answer was written in 2010. Cassandra has now become one of the leading NoSQL data stores. It is reliable, mature and well supported by Datastax. It has changed significantly. Super-columns are gone, secondary indices are in, massive improvement is done using JNI, it now has a SQL-like query language and integrates well with distributed computing tools such as Spark, and Hadoop. With time came other alternatives too; checkout ScyllaDB.
Not sure if I can convince you. But, I am working on a project that uses Cassandra. Cassandra is not the complete solution but it is very fast and it is good for grouped information.
We have off loaded all the intensive read-write data to Cassandra, and the data that are lesser in-demand and do need relational integrity are still in MySQL (on top of which there is MemcacheD). And, I guess Facebook must also be having an amalgam of MySQL, Cassandra, MemcacheD. At least that's what I guess.
To answer your questions (on my short experience with Cassandra and MySQL)
We are just starting with Cassandra, I hope someone can point out if anything above they do not find correct. I would be glad to retest and rectify, if necessary.
When I started I did not find much documentation, but now looks like Apache Cassandra page has quite a few articles listed. Refer: