Setting up Office 2011 in general was actually pretty smooth for me. Granted, I don't have my Office install tricked out with custom plug-ins or anything else out of the ordinary, so it was uneventful. My email was all living on an Exchange server, so switching clients wasn't really painful either.
Upgraders should note that Outlook drops support for Exchange 2003 and early versions of 2007 (specifically, Outlook 2011 works with an Exchange 2007 server running SP1 RU4 or later); if your IT department has been holding on to old server versions, you may be out of luck, but most Exchange 2007 and all 2010 configurations will work fine. Microsoft's also adjusted the 2011 installer so that it doesn't wipe out your 2008 install, so if you've got Entourage now and want to stick with it, you may.
I did find that as an application, Outlook is noticeably snappier than its predecessor to do all sorts of things. I was able to navigate and sort through local mail in a timely fashion, and even if I was searching outside of Outlook, Spotlight did a good job of finding the email I was looking for.
To start off on new features, we have the ribbon. Well, we have a version of the ribbon -- the OS X version is not quite the same as the Windows version ribbon, so even though I've spent time with that version, this was still an adjustment. TJ Luoma gave his thoughts on the ribbon in his Word 2011 article. I don't like the ribbon, and here's why: I have a 13" screen on my personal laptop (and my work machine is an only slightly larger 15" screen), and I need efficient use of my limited space. I want my windows and applications to be wider than they are tall, like my screen.
I liked that in Office 2008 I could open a palette and have the tools on the side of my application. Quite honestly it's about the only thing I liked, so cramming everything further down the screen for another row of toolbar nonsense kind of bugs me. I'm trying to be open-minded about it, but I have spent a couple of weeks using this new interface, and I still don't like all that stuff at the top. It seems busy and not Mac-like.
Aside from the ribbon, the biggest change to the Office email client I have seen so far is one that most people won't ever actually see: the backend. Instead of that single giant mysterious blob of an Entourage database, there is now an index separate from the actual files, so if something goes bad, it might not take your entire mail archive down with it. Like most database-driven applications, your archive is only as good as the data, so if you're importing some flaky info from Entourage, don't expect it to improve in Outlook. On the upside, now that most of your mail is stored as individual files, Time Machine backups are possible, so at least you have a simple way to back up all that data.
One of my primary annoyances with the Office email client is the search feature. It defaults to only searching the folder you are looking at. As of now I can not find a way to default the search to All Mail, All The Time. Once you set it to All Mail, if you switch to a different folder and switch back, you have to reset your search to All Mail again. Now I'm not the most popular person, but I do get a fair amount of email, and I need to archive a fair amount of it. This makes searching an important feature I use many times a day, so having it behave this way is aggravating.
Smaller irritations include the public folder subscription setup. It's a binary system: either you can see it because you're subscribed, or you can't see it because you're not. There's no Entourage-style browsing. Another small thing I didn't like was the lack of control over recently used addresses. You can't edit batches of them or disable the feature, so that time you typed in the wrong email address will haunt you forever, unless you delete bad addresses individually.
A "love it or leave it" feature in the new version of Outlook is the Unified Inbox, similar to the one that came with iOS 4. All the email from all your addresses in one place. I know people who adore this feature, but I find that after spending thousands of years (in Internet time) using different workflows for all my different email addresses, I like to keep them separate. Also like the iOS 4 version of Mail, you have an optional conversation view. This is another feature I don't like, because you can't really preview all the unread messages, and if there's more than one unread reply, you don't go to the oldest unread message but the newest.
This isn't to say I didn't find improvements in the new version of Outlook. I love the expanded Quick Look support. Click an attachment, hit the spacebar, and your attachment will probably show up just fine. I've done this with .wav files, email messages sent as attachments, Word docs, PDFs, and a variety of others. It's really nice not to have to launch another program just to get a small piece of information from a file. One of the other handy features of Outlook 2011 is PST support, which is fantastic news for switchers who have painstakingly archived gigabytes of data in a multitude of PST files. If you are an Outlook Windows user, you'll also be glad to know that Tasks and Notes will (finally) sync back to an Exchange server, if you're using one.
As scheduling goes, the meeting invitation has gotten a nice polish. Now when you receive an invite, you get a peek at your calendar in a preview down the side of your message so you can tell quickly if you have a conflict.
Another Outlook feature I like is the improvement of the alert application called My Day. I absolutely hated the last version; it was very large and seemed over-designed. Even demoting it to the menu bar didn't seem to help it much. This new version is a bit more streamlined, and with the nicer menu bar toggle it's even easier to use, and more importantly, hide.. I actually found myself liking it, which I never could have said about the last version.
Overall I find that Outlook is nice, but I am not going to switch to it at home. I don't hate it the way David Pogue hates it, but I'm not using anywhere near the number of features he's using. I spend time on email at my day job, but I don't have a need beyond the basic functionality, so it works well for me.
Given that Mail, Address Book and iCal in 10.6 are savvy clients to the same versions of Exchange that Outlook supports, why not go with the Apple apps? Well, the answer is "it depends." Mail and iCal are handy for a single user but they don't have the mailbox/calendar delegation support that Outlook offers to handle complex office dynamics. The .PST support is also not present in Mail; server-side rules aren't in either product yet, but Outlook is scheduled to get them soon. Outlook's handling of calendar invites and meeting conflicts is probably better than iCal's for most scenarios. Syncing appears to be an uncertain proposition at the moment - Outlook's Sync Services support works with Address Book now but not with calendars, although that's coming.
If you're setting up shop with your Exchange account for the first time, you might give Mail and iCal a go and see how you do before stepping up to Outlook as a personal info management system. But if you have a lot of Windows-using co-workers, or you currently use Entourage 2008/Outlook for Windows, you're almost certainly going to want the familiarity and capable features of Outlook 2011.
As TJ noted in his Word review, there are a few new adjustments to the licensing for Office 2011, the biggest being that now your registration is tied to a single machine. If you live in a multi-Mac home, this could get spendy. If you're working for SomeBigCo with an existing Office licensing agreement, check with your IT or HR folks to see if you're eligible to get inexpensive Office for your home Mac via the Software Assurance Home Use Program, a very affordable option. As of right now the program is still listing Office 2008 for Mac, but that's likely to update soon.