If you go to the phone shop by the petrol station on the main drag in Santa Eulalia the girl speaks good english. I bought a dongle late last year and on several short trips back to Ibiza over winter I went in the shop, paid 19e and she set my dongle up. You get 1gb for 19e but you must use it in 1 week so its fine for me. The daily rate option is pointless, they also do a 1gb for about 50e which you can use over 90 days but I like the 19e one. I am flying to Ibiza monday for 10 days so will recharge the dongle mid holiday but in the summer when I am there for 6 weeks I will just use my local bar that has free wifi and cheap beers!
Whatever you do, forget downloading video etc, its fine for emails, ebay blah blah but nothing to big.