exchange 2010 has been criticised for appearing to be slow for outlook 2003 clients, that's another way of saying that customers didn't rush out and update outlook to the latest versions as they became available.
Outlook 2003 users would complain that messages would only appear on a minute boundary, and generally they hand held derives would receive new message notifications before they did.
Outlook 2003 used UDP notifications from the Exchange server to notify clients of messages as they arrived, note that the biggest impact here is users running in online mode, not cached mode. Users with very large mailboxes running in online mode and/or terminal services users would have been the worst affected.
That, along with the news of Exchange 2010 service pack 2 and the GAL segmentation improvements coming later this year, along with a number of other posts on the Exchange teams website, has made it an interesting week in the Exchange community.