We have been using clustrix at AOL for quite a while. You can check their site for our testimonial. We find it to be reliable and easy to use. It requires so little care and feeding we haven't had a full time DBA on our application for several months. We've done consolidations from a sharded MySQL environment to clustrix using nothing more complex than MySQL replication. We also been able to make online schema changes without locking the DB on very large tables in excess of 400 million rows. Support is quite proactive as well both for regular release upgrades as well as features additions and minor bug fixes. We've been in production for more than 2 years and other than minor issues Clustrix has been trouble free.
Clustrix is legit. Running it in production now (on their all-SSD hardware, not the new AWS version) across two physical sites. Functionality is sufficient, performance is admirable, support is top shelf.
Their bet was contrarian: embracing SQL -- specifically MySQL wire & dialect compatibility -- at a time when the open source options were trending toward NoSQL.
For funded start-ups with a calculable data-to-revenue ratio, Clustrix buys you the option for arbitrary future expansion while retaining the functional benefits of the RDBMS stack.
Once your product succeeds to a level where daily operational reporting is necessary, you'll appreciate the ubiquity of SQL: analysts and off-the-shelf tools all speak SQL without the overhead of glue code, data staging, or ETL.
We use it on a large Rails e-commerce site and it works well. All features work as advertised and it is a drop in replacement for mysql. Their support team is also great and is like having your own dba team.
I can also say that I've been participating in the private beta of this AWS version for a week now and it is both stable and performant. Although we use their appliance in production, I'm very excited about this AWS version as it allows us to run clustrix in our testing and staging environments (where we previously had to run mysql).
I think it's a really great thing that more people/companies will be able to use their awesome database without having to purchase an appliance.
We've been using Clustrix in production for two years at TheLadders. Very satisfied with performance, wide feature set, low maintenance cost. Saved money on: not doing sharding; eliminating a full-time DBA position; repurposing hardware used for MySQL slave farm and storage. Clustrix support is excellent.