When we went last Sept. they offered EBCI just before we left and we jumped at it. For the first time, ever, we finally got A (both ways). We leave from ONT and fly into MCO, both pretty busy airports.
Previously when we've flown SW (we had switched to Delta since we can pick our seats but since they started charging for luggage, we have switched back) we have never gotten anything better than "B". And I always check-in right at the 24 hour mark. Never got an "A".
Another thing to consider, especially for those of you in the middle of the country, if you are joining a flight already in progress everyone continuing on will be able to check-in before you. For example, say we leave ONT at 8 a.m. PST and arrive in Nashville at 1 p.m. then onto MCO. I can check in, for both flights, at 8 a.m. the day before. If you are boarding in Nashville then you can't check in for 3 more hours. So everyone who is connecting through Nashville will be able to check in before you therefore diminishing your chances of getting "A". Make sense? So, if you are leaving from one of the airports that SW does a lot of connections through, you definitely should consider EBCI.
When we considered whether or not to do EBCI, we figured doing it from MCO to ONT was a no-brainer since MCO is so busy, plus I would have to rely on my DSis to check us in at the 24 hour mark. Since we were doing it one way we decided to go ahead and do it both. The $60 extra (3 of us) still made our tickets a good deal.
One thing I wish they would make one change (maybe they have??) is to allow you to use SW credits (we had some when our ticket prices dropped) for the $10 EBCI fee.
BTW, we are going to SF next week from ONT but we are not doing EBCI. The flight is only 1-1/4 hours so even if one of us can't sit with the other 2, it's a short flight. The $20 add-on would be a substantial % of our ticket price ($98/r.t.) amounting to 20%. Flights from ONT to MCO run about $250 or so so then the add-on is only about 10%.
Love that all the resorts will be doing SW Airline resort check in. Now that we've used that service (first on Delta, now on SW) I've really gotten to count on it.

So nice to not have to lug all our luggage around from the car rental return to check-in. Saves so much time and aggravation. I plan carefully and make sure we have some spare clothing just in case (read of a guest who's DD got sick and they couldn't access their suitcases-had to buy her new clothes.).
I think a lot of whether or not you should get EBCI depends on your airport. Leaving form MCO I plan on always using it.
