What database and setup can handle several million inserts per minute? We have come to a point where for a few of our customers' systems, at peak times, events are generated at that crazy rate. That is, millions of new events per minute (between 1M and 2M).

The records/docs size are 1.5Kb on average.

Is there a database that can handle such radical write speed out there (16K - 32k inserts per second)?

There are several candidates out there, Cassandra, Hive, Hadoop, HBase, Riak and perhaps a few others, but I would like to know from someone else's experience and not just quoting from each database system's website self testimonials.

If you have handled such load at your work, your advice is highly appreciated.
Ok, let me start simple: My old trusted 5 year old main db server (2x quad core AMD Opteron 2378) handles around 80,000 inserts per second, though small ones. That is without SSD for log and data on a SQL Server.

There are plenty of databases that can meet your requirement - it is MOSTLY a question of appropriate hardware. Heck, if you would go Oracle, then the smallest Exadata in a performance configuration (pure SSD) likely can do 10 times as much - we got in a project 5 gigabyte/second stable IO out of it, with a paltry 3 storage nodes.

That said, you WILL have to test it yourself. A lot depends on your programming or lack thereof. If you hit 1 insert per transaction, that is VERY different - and a lot harder to do (not at all because your log partitions need to handle 5 million IOPS then or fake the flush).