My only concern is that you then end up with a 'non-standard' datastore. 99% of datastores in use either Oracle , MySql or Sql-Server (Microsoft of Sybase). While this may be a good solution for now, will the client be able to get people to maintain the solution if / when you leave the project?

Yes , I am aware that if you extend this argument there would be no innovation at all (always use existing solutions). But it is worth evaluating and making a decision on the full cost over the lifetime of the system.

Paul , Technology in Plain English (http://www.firstpartners.net/blog)