Is the BART extension to San Francisco Int'l Airport up and running yet? Is it a quick and convenient way to get downtown (Union Sq area), better than the numerous shuttle services?

Yes and yes.
Check recent posts for additional details.

Thanks. The shared ride shuttles are OK, but sometimes you have to tour the whole city before you get to your hotel!

The fare is under $5 (cheaper than the shuttles) and BART is MUCH faster, of course. It's a win-win situation as long as you don't mind carrying your luggage on the train.

Several comments in SF Sunday Chronicle about Bart and SFO.
Main subjects were time frame and luggage:
Be sure to check the schedule as to how early in AM Bart can get you there and still have time for the required "be there before the flight leaves".time frame..Going from SFO to downtown is a definite YES
Luggage:
Even though there is a space at each end of the cars, this is really for handicaped use. If a wheelchair needs the space you would just have to move luggage to ???? Bart officials are working on this problem.
Most other comments seemed positive especially the cost...
try it, you'll like it...

As long as you don't have a ton of luggage - and if you did you wouldn't be able to get it onto the train to begin with - you can just keep your luggage right with you, either on the seat or floor beside you.

This past weekend I actually took BART from Oakland Airport, where I flew into, to SFO, where I was meeting my brother. The BART train went through San Francisco on the route.

I didn't have alot of luggage and I was traveling at non-rush hour times, but I found it incredibly easy and fast. This is especially true at SFO where BART goes directly into the airport. No need to take a shuttle bus like on the Oakland end.

Go to BART's website and download a time schedule. On weekdays their service starts up pretty early if that's a concern.

Thanks for the great BART advice - I think I'll bid the shuttles adieu! A great city like San Francisco deserves a great mass transit link to its airport.

Beware that BART doesn't go to the airport on Sundays until about 7:30. If you allow 2.5 hours before your flight to get there, check in, and allow for delays, that means that your flight must not be before 10:00 am.

There is a Sam Trans bus (website available) which runs earlier and is cheaper.