I use Outlook 2007 as I already had the full Office suite and therefore it seem silly to ignore it. Generally speaking I think Outlook is about the best email program going (and I'm not generally a big MS fanboy) and the Calendar and Appointment features come in very handy at reminding me where I should be

But Outlook is very expensive to buy on it's own or to upgrade the version of Office you buy for it. Therefore I would not be tempted to pay for it and would stick to WLM etc if I did not already have a copy.
Mark.