If you're thinking of implementing OWA, don't think 'either/or', but rather 'both/and'. OWA and Outlook both have distinct advantages, but both also come with major issues.
OWA is great for those on the road, but it doesn't support 1/10th of thefeatures of regular outlook (such as e-mail notification, spell check, most outlook forms, etc). OWA can sometimes get confused when you try to open more than one message at a time. However, it has a much smaller memory footprint that regular outlook. The OWA supplied with Exchange 2000 is DHTML/DOM compliant, and will fix many problems, but I won't hold my breath. My tradeoff solution is to use IMAP/LDAP support with Outlook Express, rather than deal with the quirks of Outlook Web Access.
ObInfo: A few months ago, our exchange private store got corrupted. OWA could access many messages that regular outlook wouldn't touch.