This may be an interesting statement coming from my side, but...
I generally don't use Outlook all that much, I'm a Mac die-hard (and I'll convince anyone, if needed, that that's really the best way to stop worrying why your computer does certain things and get on with your life, turning it into what it's supposed to be, A Tool That Works [and is pretty at the same time

]) and Apple Mail, iCal, SWA (soon to be in Safari, we use that already, internally), plus a couple mobile devices (iPhone for sure, but I have a Blackberry as well) usually do the trick for me.
I did give SmartCache a couple of tries when it was brand-new (during the Beta and initial 11.0 deployment phase) and must say that it didn't work too well for me. Problems were partially the typical type of a new product in terms of stability and general bugginess, but also somehow bound to the size of my mailbox (probably was at around 6 GB at the time, adding about 1,5 GB/year, so go figure) leading to infinite pain in terms of hangs and issues, especially when doing bulk operations (like moving a couple 1000 messages at a time).
I understood that most customer's usecases were not as heavy and complex as mine, got feedback that it worked quite well (or OK++ for many people) and ... gave up on it.
Well, I'm happy to report that it's back on my desk now, at least during daytime working hours, as part of a Windows Vista/Outlook 2007/Internet Explorer 8 setup that I use to confirm the quality of our pure-Windows users user experience. The hardware this runs on is still a Mac (A Mini with Bootcamp on it, I just like the keyboards and screens and everything...

), but software-wise it's a genuine Wintel-Microsoft box, grounds up. And .... it works extremely well.
Granted, I really don't like Office 2007's UI (who does?) - although Outlook is probably the most "unharmed" product in the package, I really cannot get my head around what they did to mess with Word, Excel and Powerpoint.
But then.... it's fast, reliable, doesn't crash, search works now including Windows Desktop Search, Smartcache manages my line performanace and connection (my desk is in Frankfurt, Germany, the Scalix server I'm connected to is in Ottawa, Canada, around 250ms latency) and I can do things like moving 6000 message in one go from one folder to the other. It thinks for a short moment, displays a progress bar (that's Outlook's part, actually), then says it's done, thendo the sync magic at really low bandwidth in the background.
SmartCache works for me and I would recommend it for most people in most situations, unless you're really in a super-highspeed-lan-office, but even then, using it offloads the server and will give you significantly better scaleability, overall.
It's brilliant. (watched too much Top Gear recently...) I'm sold.
And you can quote me on all of the above....

Florian
P.S. I hate IE8. It doesn't seem to work with this forum page and I am typing this in a up-down-flickering text box. Maybe a way to make people keep their posts short so that they fit on a single screen. It sucks. I'll install Safari right now.