Maybe it's time to look at Mozilla Sunbird 0.8 for Mac OS X. It's developed by the same people who brought us Firefox and Thunderbird. It's almost identical to the Lightning calendar plugin for Thunderbird, but doesn't require that email app to run. Upon first startup, Sunbird will import events and tasks from your existing calendar application.
The UI is plain and simple, probably because this is a multi-platform application (Windows, Linux, and Solaris in addition to Mac OS X) that shares a lot of code between the different flavors. Sunbird can tie into CalDAV servers such as the one in Mac OS X Server 10.5, and can subscribe to any .ics format shared calendars. In limited testing, Sunbird felt responsive in searches and was quite stable for betaware.
Sunbird, of course, is free. If you are searching for a new calendar app, give it a try.