The first thing to understand about SharePoint calendars is that they are not trying to replace your personal calendar.

The Calendar list is just a list that happens to have a start and end date. SharePoint provides a view that can work with any SharePoint list where you can identify 2 date fields that can be used for start and end times (same requirements for the Gantt view).

SharePoint provides open access to all SharePoint list data through standard interfaces such as XML web services and the SharePoint Stssync Handler.

Outlook makes use of this access to render the SharePoint data in the familiar Outlook interface. This provides an advantage over the web interface:

• Side-by-side and overlay viewing with personal and/or other users calendars + SharePoint Calendars
  

Because this is Outlook client functionality, it is not dependant on an Exchange server, it means that:

1. The free-busy service is not aware of the SharePoint calendar item when scheduling a meeting
2. A .pst file is required on the client to store the SharePoint calendar for off-line viewing/editing
3. The SharePoint calendar items will not appear on a mobile device
4. You may have to open lots of SharePoint calendars to get the "big picture"

Some people may claim that these issues make the SharePoint Calendar feature "essentially useless" (even though when they try and demonstrate the features that fail, they work ;-). One should really say that it may make the non-browser functionality "essentially useless" for a very small number of users. Let's focus on how the majority of users can still get great benefit from these features.

1. The team calendar is not meant to contain my personal calendar items. However, if I do want to include an item from a shared calendar in my personal schedule (say and industry or training event), then I can just drag and drop it from the side-by-side view into my calendar. Now it will appear in my free-busy information and others will see that I have reserved this time.
2. It is the default behaviour that PST files are allowed so this will work for most users. I do know that some organisations are blocking PST files as they start to introduce better email management and archiving solutions but this does not have prevent SharePoint PST's from working (see http://technet.microsoft.com/en-us/library/bb851502(EXCHG.80).aspx Create a group policy that sets the following registry subkey to a value of 1. This setting prevents users from adding anything new to .pst files. Users can still create new .pst files but they cannot add anything to them. This setting blocks only Microsoft Outlook .pst files. It allows Microsoft SharePoint .pst files to be connected and updated in a user's Outlook profile.)
3. True, but (1) solves this for the ultra important items that you took the split second to add to your calendar by drag and drop. For general access, if your SharePoint is exposed through an Extranet, you can use the mobile access view built into SharePoint and that great 3G functionality in your Windows Mobile 6 smart phone.
4. This comes down to implementation. I always try and encourage my customers to go with a single SharePoint events calendar for the intranet, which lets every user easily see a global view of things going on which is very handy when you are "just thinking about planning something at some time". Only use team calendars for project related milestones (use tasks for deliverables rather than calendar items, I have another big post coming on this). You can use the Content Query or search web part to rollup these multiple calendars for consolidated views (which also provides a single RSS feed for the many calendars).

An extra benefit Outlook provides to the SharePoint Calendar is richer printing. Don't forget to check out the free Calendar Printing Assistant for Microsoft Office Outlook 2007 http://office.microsoft.com/en-us/outlook/ha101687211033.aspx

My biggest complaint about SharePoint calendars: no color coding (yes there are 3^rd party solutions for this)

I also have some other SharePoint/Outlook tips for a better meeting room booking solution and how to find your appointments that have associated meeting workspaces here http://www.wssdemo.com/Pages/meeting.aspx (although written for WSS v2 it is still relevant today).