I sure think so!!
Why? Well I think this simple math lesson should demonstrate it pretty inaccurately:
1. Thunderbird + nothing = still better than Outlook (resource wise, and you don't have to pay for it).
2. Thunderbird + Lightning = all the functionality of outlook. Or at least all the functions of outlook that I ever used (Seriously, who journals in outlook?). Oh yeah, and it is still free.
3. Thunderbird + Lightning + Provider = The same as #2. So this bullet is completely useless
4. Thunderbird + Lightning + Provider + gCal = Freakin sweetness!
Seriously, do you need any more proof?
Okay, well for the people who don't want to blindly believe the truth of what I speak, ask yourself this: What is the worst thing about google calendar?
Answer: You can't view or edit it when you are w/out internet connectivity (because it's all online only and stuff). But with this winning combination you CAN view and edit offline, because it is stored in your Thunderbird client!
Plus, if your computer takes a flying nose-dive into microsoft stupidity, your calendar info won't be lost because it is totally still available ONLINE!
Oh yes, pure sweetness!