While Exchange becomes better and better at handling large mailboxes and more importantly being able to recover from disasters should they happen, it's time that Outlook starts catching up and offering MUCH better support for larger mailboxes. Performance beyond the 1 GB mark has always been a bit iffy but much more so when Office 2007 RTM'd.
New code in Service Pack 2 makes life MUCH more bearable, but wait! It's available in separate patches as well, meaning you won't have to wait for SP2 to release before having the benefit.
After installing you need to give Outlook a moment or 3 to adjust the contents of your files to the newer format (suggest not to switch off right then).
There's a really nice matrix summarizing the performance gains here: http://msexchangeteam.com/archive/2009/03/24/450881.aspx and the patch is available for separate download here: http://support.microsoft.com/kb/968009/.
Performance metrics are published around the 5, 10 and above 25 GB mark, although I think having a 25 GB PST or OST is RATHER brave, especially since we all know that PST's are nasty when they break and OST's on laptops at 25 GB's....... enough said!
This brings back the argument for high availability and the perceived cost being to high. CCR drops the barrier to entry on highly available mailboxes, and while I'm aware that disks are never as cheap as when YOU have to buy them, Direct Attached Storage (DAS) is considerably cheaper than SAN storage - which is normally perceived to be the barrier to entry on any kind of clustering.
CCR is different to traditional (SCC) clustering since the hardware may be dissimilar (different vendors) as well as having cheaper storage - DAS and not SAN storage becoming a major option.
If you'd like to look at something you can take to the bank or the finance department 😉 I suggest you checkout Missy's white paper on the subject. She outlines where the costs are and makes a stunningly good argument for CCR, especially when it comes right down to it: where do you spend your hard earned cash! Continuous Cluster Replication and Direct Attached Storage: High Availability without Breaking the Bank
While this patch DOES make life simpler and more bearable for those of us who DO stay mobile with a lot of mail, or even desktop users in remote offices who depend on cached mode to get ANY mail, it should not be more of an excuse to slack off on mailbox limits if you're not running some form of high availability. I'm hoping that a combination of more High Availability (HA) options in Exchange 2007 as well as better code in the Outlook client will allow you to service the need for larger mailboxes as well as securing the mailboxes themselves.