We were very happy with our stay at Duque De Loule. It is very well located both with respect to Porto and to Gaia. There were some good restaurants on the same street and others we found online were only a short walk away, as were most of the attractions we visited. The street is quiet at night and we did not have problems with noise. We really appreciated the water machine!
Only complaints would be that the bed is a little hard for our liking and would have appreciated a couple extra pillows, although admittedly we could have asked for these but did not. Also the apartment is two floors up and only accessible by stairs, which did not bother us but could be a problem for those with mobility issues!