I was in Porto in Summer 2005 and I find the quality of the stations and trains were great! Most of the stations of the downtown have natural light and are very high and with a lot of light.
I loved the system to get into the stations with card detectors.
The only problem is that they were doing important works on the network (line over Douro was not completed yet, as well as the stations near Avenida de los Aliados) and system was not working on the weekends.
I don't know if anyone said it, but some of the stations are designed by Soto de Moura, great portuguese architect.
Very beautifull trains and the construction seems cool, though most of the stations looks pretty ugly, anyway the fact is that this is not a subway , it seems more like a tram with a few subway stations.
Amazing concept!! Having a metro and a tram network as one! That is pure brains! You have a great forward thinking government!