I don't really get too into new calendaring programs since we use Meeting Maker at work and I don't know of any easy way to get another calendaring client to interface with Meeting Maker. I do, however, play with other calendaring products from time to time just to get a feel for what is out there. Lately, I have been playing around with Lightning, Mozilla's calendaring add-on for Thunderbird. Lightning is the sister product to Sunbird, Mozilla's standalone calendaring application.
I think the basic idea of Lightning is to make Thunderbird more of an Entourage/Outlook-like product. I really like the idea of this, as I think that there should be a tight coupling between mail, calendaring, and contacts management, but I don't think this product is quite there yet. But, it's only in version .7.
One cool thing that I noticed about Lightning is that it can handle ical (.ics) files sent to Thunderbird. If you get one of these and open it, Lightning will prompt you about adding the event to your calendar. I assume that Sunbird does the same, but I have not tried it out yet. I'm also running the Provider for Google Calendar add-on that allows Lightning to interface with my Google Calendar.
If you're in the market for a new calendaring solution and you are an early-adopter type, you may like Lightning. If you want a polished, ready-for-market product, this is probably not it.