Greetings,
I started using Thunderbird several months ago at home, as I usually 'guinea-pig' software I'm considering installing on user's machines at the office. Based on my experience I'm in the process of migrating my users to Thunderbird (from OE) because it's cost effective, has more and better address book options, is a lot more secure and deals with attachments better. (I've never used Outlook as we don't utilize an Exchange server). I spend less time solving my user's email problems now.
My users who have Thunderbird seem pretty happy with it - certainly none of them miss OE. My average user doesn't seem to notice any real difference, and from my perspective, that's a GOOD thing.
The only problems I have with it in the office occur when I miss something in configuring it - there are a LOT more configuration options with it than with OE, and their access is not quite as intuitive.
I also use the Sunbird calendar. Sunbird is available as either an extension to Thunderbird or as a stand-alone executable. It works pretty well for sharing information through webDAV, which I have set up on our intranet (Apache) server. It does not have notes but does have a task list, which I use for both purposes.
Sunbird is a relativly young project, and there are still some bugs, but it's growing quickly enough and has become useable enough that I've been installing that at the office for some users as well.
Hope this is helpful.
Greg