I just read this 2-part series from the Acunu blog.
Cassandra under heavy write load, Part I
Cassandra under heavy write load, Part II
Acunu has some really interesting new tech to underlie Cassandra with some great results for inserts and ranged queries under load.
My personal takeaway, I didn't realize that Cassandra is write-optimized so heavily. I think this would be a very important consideration. You always need to match your DB to the kind of load you are expecting. Whether you're write-heayy, read-heavy, in a balanced scenario, you need to make sure your DB is tuned and scaled for it.
In this particular testing scenario, the focus was on ranged queries as it appears that normal reads are not affected by substantial write bursts. If you're relying heavily on ranged queries however, you really need to understand how your app is going to be affected due to Cassandra trying to perform SSTable compaction under load. It looks to be a significant impact on response time.
Acunu is definitely worth a look if Cassandra is your weapon of choice. Along with that, check out the latest updates for Cassandra. It looks like the team is also actively focused on making Cassandra a better and faster product with each iteration.