Please read this article
Class design considerations - howto part 10
about my view on object modelling.
While I create this blog to get a chance to express my personal opinions on things (the intention was to try to keep my regular aspcode.net site as correct as possible) I couldn't help ranting about class design in that article.
In short - I believe that object modelling is a good thing, no question about it, but I have been in projects where people sit at meetings for days discussing it, instead of get going. You can't model the whole world excactly as it looks - aim to create a model to cover your requirements.