Its good feedback and good ideas, I've made notes about it for future development. I think any solution like this is going to have some limitations. It could work for 1000 rows, but at some point the amount of data will be too much to import/export from a web page. I'm not sure what the upper practical limit is, maybe 10,000, certainly a million rows is too much to try and import/export from a web page reliably. Maybe a dektop app that could access the same databases could handle much more as it can run for a long time.
So it may be better to develop a set of tools as dlls with a WinForms GUI. The same dlls could be used in the web page for smaller data sets.
It has been suggested before that I should implement tools to import/migrate users and content from DotNetNuke. There is an xml format for importing/exporting blog posts as blogml, which would make it easier to migrate blogs from a variety of other blog systems.
These are all great ideas, eventually I will get to them if no-one else does, but I have a lot of ambituos plans on my agenda, so there are also opportunites for 3rd parties to develop and market tools such as these as well.