Thoroughly recommend Hugo's place! The location is fabulous: close to everything you could possibly need, but on the top floor so it's quiet and you get a great view from the kitchen, over the city towards the ocean sunsets. The metro is indeed virtually on the doorstep, or if (like me!) you like to use your legs, you can walk to the centre of Porto in less than half an hour. The apartment itself is clean and modern, and perhaps most importantly, Hugo and Alex were fantastic hosts: very welcoming, always prepared to answer my questions and share their local knowledge, and genuinely concerned that I should have a comfortable and enjoyable stay. Oh, and being treated to freshly-made (American-style) pancakes every morning was a delight! :-)
A couple of things to be aware of, in case they matter to you:
The room I was staying in (the one in the photos) didn't have a window. Not a problem for me, but if having a window is really important to you, do mention this to Hugo, as he may be able to put you in a different room.
There is no lock on the bathroom door. This was absolutely never a problem, but if that sort of thing freaks you out, then at least you now know in advance! The bedroom door does lock, however.