The apartment is okay, good size for a party of four if you aren't planning on cooking too much (kitchen is small but has everything you need).
The location is absolutely perfect - but you will get woken up by the cathedral bells every half an hour from 8am onwards. Definitely worth it for the view. You're right next to lots of nice restaurants, cafes, the river, historical sites and shops - head to Fabrica or Fedaral nearby for amazing brunch!
Having said that, it wasn't the cleanest on our arrival and all the glasses and plates were dirty (looked like they'd been washed in a not-working-very-well dishwasher). The air con doesn't do much to cool the apartment down (it's a small aircon unit in the living room) but it's bearable. The bedrooms have fans. The upper bedroom fan in particular is very pathetic!
This wouldn't have been a major issue for us really except the host then tried to charge us an additional cleaning fee for apparently not leaving the apartment clean - bearing in mind we didn't eat in the apartment at all, the 'mess' we left amounted to unmade beds, glasses in the sink and rubbish in the kitchen bin. There's already a cleaning fee added to the booking and we were not told that we had to stack the dishwasher or take out the bins. Initially the host claimed to have photos showing the mess. After we argued that the apartment wasn't actually even clean when we arrived and we weren't given any instructions on how to leave the apartment, the host tried to reduce the additional cleaning fee. We then asked to see the photos which didn't appear and the host agreed to waive the additional fee. Not a pleasant ending to an otherwise acceptable stay.