At ShermansTravel, we already tested Cassandra and Tokyo Cabinet/Tokyo Tyrant. We are starting the implementation phase this week. The issue with Tokyo Cabinet for us was writes were too slow. The issue with Cassandra was that reads were a little slower than we'd like, but with some caching (either eventually being built into Cassandra or with memcache in front of it).
I'd love to get a better feel for who is using Cassandra and what kind
of applications it is seeing. If you are using Cassandra, could you
share what you're using it for and what stage you are at with it
(evaluation / testing / production)? Also, what alternatives you
evaluated/are evaluating would be useful. Finally, feel free to throw
in "I'd love to use Cassandra if only it did X" wishes. :)
I can start: Rackspace is using Cassandra for stats collection
(testing, almost production) and as a backend for the Mail & Apps
division (early testing). We evaluated HBase, Hypertable, dynomite,
and Voldemort as well.
(If you're in stealth mode or don't want to say anything in public,
feel free to reply to me privately and I will keep it off the record.)