Telmo's apartment is just perfect. The place itself was very easy to get to from the airport. The metro stop (General Torres) is just in front of the apartment building.
Walking to the center of town was also easy because the apartment is just across the bridge to Sao Bento, which is where most of the major sights and restaurants are.
There's also a fairly well stocked supermarket close by (next to the Monastery).
I would highly recommend Telmo's apartment especially to those visiting Porto for the first time.