We stayed at Domum last week and I think the best thing about this apart hotel is it's location. It's really easy to get to most of the attractions on Porto from here. The rooms were modern and the beds were comfortable, Sergio who checked us in and out of the apartment was good and welcoming.
The rooms/apartment that we were checked into was smaller than we were expecting which we were slightly disappointed by, we were there for a week though, I'm sure most people would go for a weekend so this probably wouldn't be a massive problem.
Something that would improve it would be air conditioning, we were at the front of the building and had to have the windows open at night which sometimes meant we were woken up by noise outside. There wasn't a lot of easily accessible storage space it the room, to hang shirts, et cetera you had to climb into the bed which might not be great for the less able, also there is no lift, it's probably worth pointing out that Porto itself is extremely hilly so maybe reconsider going if that would be a problem although it's a great city and would be a shame to miss it.
It's worth pointing out that the 602 airport bus stops really close to the apartment, google maps directs you by metro and doesn't mention the bus but there is a bit of a walk from the station. Parking near the apartments is very difficult but I think that's the same throughout the entire city and not just related to this location.
There is a good little bar and cafe at the bottom of the street which is good food and cheap for breakfast or first evening drink and a supermarket around the corner.
All in all I'd say domum is fine. It depends on your expectations if you want real luxury go somewhere else but a well priced apartment in a reasonably central location it is. It's a good 10 minute walk to the real heart of the city but we didn't find this a problem.