I never use add on air BUT then again have never flown to England or Europe for a cruise.
Sometimes it can be a very secure feeling to know that the ones you are paying are the ones that have to cover your butt. Using add on air the cruise line is responsible for getting you there and covering any expenses that are incurred when something goes wrong. That's the upside.
The down side is that it is usually like flying Southwest, you usually can't get there from here, three stops, two plane changes, flying at the most inconvenient times known to man but it's not as cheap as Southwest. The only exception to that is to pay for a deviation that lets you pick your own schedule.
Add on should be able to be quoted to you by your travel agent or a Carnival customer service rep and in this case would be well worth some detective work and comparing it against what you can get on your own, especially if you are a little savvy in booking air on your own.
I would consider no less than 36 hours on the ground in Barcelona prior to your cruise, more if you want to see a bit of Barcelona before your cruise, sometimes add on air does not allow for time windows, they put you on the ground and your off to the ship, this is where deviation comes in handy, you still get the security of the cruise line being responsible but you get the flexibility of choosing your own timing.