My boyfriend and I stayed at Dar Almadina for a couple of nights last week and really enjoyed our stay.
I hadn't taken down the address of Dar Almadina and when we arrived, I realised this was an error. I called up Omar, the manager, and asked for the address - he offered to pick us up from the train station, which was very kind.
Dar Almadina is right in the Medina (about a 5 minute walk from the station) and is really well located. Walking to restaurants along by the beach takes about 10-15 minutes. The louage station (for shared minibuses to lots of destinations) is about a 10 minute taxi drive away.
The Dar itself is lovely - the courtyard where you have breakfast and each of the rooms off the courtyward, all really nice. It's generally a quiet area but the closest minerette tower for call to prayer is very nearby, so prepare to hear that quite often (including at 5.30am). The bathroom has everything you need but it's worth noting that the shower isn't a self-contained unit - it's a curtained-off area in the corner of the room. As a result, the bathroom (which is only small) can become slightly drenched!
Breakfast is standard fare - baguette, jams, biscuits, yoghurt and an egg/omelette, as well as coffee or mint tea - and very good. Omar is really accommodating and just asked what time we wished to eat, so you can do things on your own time.
The rooms are pretty cheap (50 TND per person per night in low season) and very good quality for the price.
I definitely recommend staying at Dar Almadina if you're after a quiet, pleasant and cheap stay in Sousse.