I use CUBRID for two of my company sites. I like its built-in counter functions, necessary to keep track of statistics, and the native support for HA, something other RDBMS do not provide. They have a nice architecture, which you will really like if you want to host multiple sites on one machine, and they are controlled by different network devices. They have a middleware called the broker (you can look at their architecture for more info), which you can create for each db, and each of them will listen to a port of the particular device. Quite useful implementation. Overall, the performance is quite good, especially with tuning. If you have questions on this, CUBRID has quite responsive community. You can always ask them, they are always online at least for me GMT +6. Also, they recently tweeted that they are going to release a new version these days which will be the best release so far with almost full compatibility with MySQL. It's not important for my projects as they are natively developed for CUBRID, but I believe it will be really useful for those who migrate from MySQL. I plan to run the new version on a test machine and see if there is any performance gains. If so, will upgrade, otherwise, I will stay on the current 3.0. Let's see. Anyone else want to share their experience?
