I have been using Iberbanda for three years in conjunction with Skype. For the first two I had nothing but problems but having changed to Iberbanda’s Avipack 512 things have improved. I regularly do a speed test and the download speed is always around 500 but the upload is erratic. I have found that I can use Skype with little problems.
I think the quality of the service depends on how near you are to the signal source. I am living in the campo in Alhaurin el Grande so am not too far from Marbella. If you choose to go with Iberbanda make sure you have minimum the Avipack 512 package that will give you the above speeds and a 951 tel nº with free call through out Spain.
My two neighbours both used Iberbanda but had the very basic package and had nothing with problems although they paid the same monthly fee as I do. On average I pay around €45:00 pm.
I hope the above helps. If you would like to test my system my Skype name is <global_skyper>.