Not the most ideal time to go as it can get damn hot. By August passerines have stopped singing and this, plus the heat, means that they can be hard to find. Meanwhile drier conditions can mean wildfowl are distant and heat haze can be difficult. Then again there will still be lots to see and southward migration will have started - so you'll see flamingos, BoPs, waders, gulls, terns, azure-winged magpie, etc. It's an iconic site so if you're down in southern Spain it's well worth a visit, but, as always, I'd suggest spending some time on the Cadiz side of the Guadalquivir if you have time. You can see most - if not all of the Coto's specialities here and you have better access to the mountains, it's better for raptor passage, better for some 'specialities' and has much more attractive scenery & villages. Send me a PM and I'll send my notes on the area.
I'll be out in Cadiz province for much of August and would be happy to meet up with you if possible. Apart from anything else we can drown our sorrows together given the doleful prospect of First Division football next season & Pompey in Europe (please not, but it ain't looking good today),
John