I installed DB2 Express-C on my Ubuntu box, and not only did it break itself after one day, it completely broke apt-get. I've never had that experience with any other software on Ubuntu. I tried going through internal channels and no one could point me in the right direction. I doubt that external channels would be anymore responsive.

DB2 deviates from other databases I've used with respect to syntax. Most free information is only from IBM. Compare that to how much information there is about MySQL.

pureXML is not amazingly fast. IIRC, it was orders of magnitude slower than relational queries. I believe development on pureXML was recently canceled.

Occasionally, we run into limitations of MySQL. When that happens, it usually means we're attacking the problem incorrectly. It doesn't mean we need a faster database.

I would be most interested to see examples of startups that use DB2 Express-C; I've not heard of any. When Facebook built one of the largest data systems around with MySQL+memcache and other open source software, it makes you wonder why you need "enterprise" solutions.

EDIT: disclaimer: I used to work as a developer on DB2.