One of the problems with Simyo is that it appears you can only get it online. I was meaning to buy one of their SIMs for quite a while, but was a bit tied up and ended up ordering it online about a week ago, but had to leave the country unaexpectantly before they could deliver it. I did get an email from them saying they tried to deliver but I was not there. I'll only be back next week.
So the bottom line is you can only buy it online, and need to have a Spanish NIE and a Spanish address to do so. They deliver it by courier and it takes around 2 working days. It needs to be delivered to the account holder who needs to sign for it and sign the agreement with simyo. It can therefore not be sent to a friend or to a PO box.
You can chose whether to go postpaid or prepaid but the rates are exactly the same. The only difference is with postpaid you get a monthly bill, and have to specify a Spanish bank account at the time of order. I went with prepaid, which cost me 1 euro plus 3.95 postage. The SIM comes with 5 euros credit, and you get a further 5 euros if you buy through someone who recommends you ... such as me
You also get another 5after your first topup of 10 euros. Not a bad deal really. Basically, it comes down to for 14.95 you get 25 euros of credit.
Though I do not have my SIM yet, I have used my friends plenty of times. He has it in his 3G phone hooked up to his laptop via a cable. He only uses it for data and has had it 2 months and not paid more than 5 euros a month. Not bad really!
It is a MVNO that piggybacks on Orange, whose coverage is not bad in most places, though Yoigo (with Movistar coverage) is much better, and they have more 3.5G coverage.
Hope this helps! Just ask here if you have any more questions.