It's obvious that I'm not an expert on Cassandra. So the question may sound silly.
Given an existing SQL-based project does it give any benefit or is it even possible to apply a no-SQL database(e.g. Cassandra) as an additional layer between business logic and SQL database to speed up our queries or inserts.
It's relatively new technology and I'm trying to find its place.
I would go with Windows Server AppFabric aka Velocity distributed cache with SQL Server, assuming you are on the .NET platform.
Cassandra will work fine, but if you don't care if you have to rebuild your data memcached will be faster.
But if you want a persistent cache, Cassandra is probably your best option -- reddit started by using Cassandra like this and is working on moving more functionality to it.