fyi, i used the movistar 3g dongle rental from the rentals office. Speed was ok. I was mainly replicating work mail, so did the job.
I think it was 8 euro a day.
Some of the 3G broadband providers are introducing 'pay as you go' dongles. Bit like the mobile phones.
'3' in Ireland have just launched it.
So this could be a good solution if it becomes available in Spain.