There is an option called "ColumnStore Index" available in SQL Server 2012.
Is it comparable with columnar databases such as Cassandra, HBase?
Few advantages of going with SQL Server 2012 can be:
What other factors can be considered to choose between SQL Server 2012 and other Columnar databases in case faster query performance is a requirement.
I'm not sure what you mean by "it is updatable", but in SQL Server 2012 tables that have a columnstore index cannot be updated. You must first drop the columnstore index or you must partition around the columnstore index in order to support changes to the underlying data.
Also, columnstore indexes are useful in DW systems where very large amounts of data have to be aggregated and accessed quickly.
In SQL Server 2012 you have the alternative of indexed (materialized) views.