Portugal 3G and Mobile Internet
Portugal has a good 3G network. The cost is the same with all three of the major telephone companies. €1 per hour. It's OK for light usage, but fairly expensive for those how work online, for long periods.
Note: It seems that if you topup with €15 or more Optimus convert that into more than €15 worth of credit. I'm not sure of the details, but I got €80 of credit after topping up with €60 of cash.
Coverage is very good. I used the Optimus network during Spring and Autumn 2011 and had good service.
As most young people in Portugal speak good English you should have no problem buying a SIM card or getting advice.
A potential problem for some may be obtaining credit. Unlike many countries, you cannot buy top-up credit from regular shops like a corner store. Credit is only available from phone shops or from ATMs if you have a Portuguese bank account. As I travelled a lot in fairly remote parts of the Algarve, away from the major shopping areas, I had to top-up in Optimus phone shops whenever I saw one.
I bought a Optimus SIM card from a Optimus store in the city of Porto, without any problem. I also bought one from a Phone House store in Tavira, Algarve. I already had a 3G dongle, so didn't buy any hardware from them.
The SIM card cost €14.90, and came with several hours of time pre-loaded. You setup the Kanguru SIM card online. It's in Portuguese. If you know a little Portuguese or Spanish you'll be fine. If you're not sure take your computer and dongle if you have one to the store, and get the sales assistant to register the SIM for you.
The Optimus 3G network has excellent coverage. I have had 3G data network at every place in Portugal I have tried to use it. This includes fairly remote beaches in places such as Praia do Amarda and Boca do Rio in the edges of the Algarve.
Optimus APN settings
With the new Kanguru Time SIM card you do not need APN settings.
However, If you have an older type phone and need the APN settings, here they are
User name: guest
I used an iPhone 3G and a MiFi dongle without problems in Portugal.