I think you are going to be tied to Pocket INformant for quite a while as I do not foresee any other PIM app coming close in features. It seems PIM is going in the opposite direction; there is a search for the lowest common denominator of necessary pim fields on now as more and more phone users seek cross platform 'cloud' type syncing.

I think your only hope of getting to use nokia's great hardware is if Pocket Informant comes to Symbian or Maemo.