Since nobody answered, I do it myself after my own experience!
It seems to me that Portugal for prepaid data is good enough.
As usually, getting reliable information in the shops is quite difficult, since it always seems to me that I know and can do more than the people in the shop know or dare to know...
I inquired in a Optimus shop, they had a prepaid plan at reasonable prices (22 Euros/1 GB traffic), but the lady insisted that it would not work without or I would need to buy the usual USB dongle (I am using my mobile phone also to connect my PDA and my PC, therefore this is for me a no no, when possible).
Waiting for a better answer, I went into a TMN shop (actually it was a multibrand shop, but they offered me a TMN plan as the best).
I got a data only SIM card for 15 Euro, with 10 Euro credit, which would allow 400 MB 3G traffic + (important!) 400 MB wifi traffic under ptwifi coverage (usual registration procedure required to get the password to use wifi). My hotel (and also other places and malls around town) had ptwifi, therefore this was very good, doubling the potential traffic one can use.
Buying the 25 Euro card would entitle to 1 GB 3G + unlimited wifi, but it seems you never find those as a start, and you cannot top-up to 25 Euro, you must pay 25 and loose the initial 10.
Take this into account and make your plans accordingly.
SIM cannot be topped up by Internet unless you have Portugal paying cards, at least so it seems to me.
After all this, I was online in 5 minutes, walking away back to the hotel. Speed is good and sometimes 3G was even speedier than wifi! Everything in the usual ranges, anyway. Do not expect 7 Mb/s speeds!
For 3G, traffic is decreased at 1 MB chunks, therefore be careful if you check your mail frequently in different sessions!
Also GoogleMaps on the mobile phone seems to me particularly "dangerous" under this respect. Does it open & closes sessions continuously?
All in all a reasonably good experience.
Sleepy