The calendaring on the iPhone is great; unlike Outlook you don’t have to be at your desk to get reminders of appointments and meetings. However entering calendar items on the iPhone keyboard is a pain, and the UI doesn’t expose a full range of reminder options, e.g. ‘1 week before’.
Of course one can sync between the two using iTunes, but this is tedious to say the least and not exactly practical if you use your Calendar and To Dos a lot. The iPhone OS 3.0 introduced Exchange ActiveSync, allowing medium- to enterprise-sized businesses with an Exchange Server installation to do 2-way wireless sync. For smaller enterprises this may not be an option, even if they have Exchange, due to the increased security concerns of making their mail server internet facing.
It is possible to have 2-way sync without Exchange, by using a Google Calendar as intermediate store and synching both the iPhone and Outlook against that. Internally we use ‘Google Apps for Business’ for our e-mail, mainly due to the excellent spam filtering, and that also provides calendar accounts. However any Google calendar account will do.