There are some applications for which I suspect ZODB might not be well-suited. These probably ones where there are lots of records to scan and/or lots of updates, which I presume could lead to major ZODB bloatage. I started using gadfly with zope and came to realize that the warnings regarding it's toy status were to be taken seriously. I switched to mysql and have been very happy with it. It's easy to install and maintain, uses standard SQL syntax and has very good performance. My understanding is that similar arguments can be made for postgres which has some extra features sub subselects but also its own downside.