Is creating a table dynamically at runtime based on a user\'s interactions a good or bad thing for a web-app? (I\'m talking java, but question probably applies to more).
In general no, it's not a good idea. Normally whatever they would be creating would be records within a table.
But there are exceptions, for example if you are creating a hosting account for them, with the tables that they will then use personally.
Without giving more detail on your situation I can not give a better answer than that.