Mozilla's Thunderbird 3 is a disappointment and I cannot fully recommend it at this time. Those of my clients using Thunderbird 2 should stick with it if they are using the Lightning calendar add-on.
Originally, Lightning was to become an integrated part of Thunderbird. I lost track, but at some point that plan must have changed. The Lightning Project remains intact as a separate entity and Lightning remains an add-on.
Here's the problem: Lightning 0.9 won't work with TB3 and Lightning 1.0 missed its deadline. A month after TB3's release Lightning remains in beta.
I was excited by the prospect of a serious and economic competitor to Outlook. TB3 is not yet there.