I don't think you could find a more central location in Bergamo - the street next door to the beautiful main square and close to all the restaurants, bars and shops. Nothing in the upper town is more than a ten minute walk away - including both the funicular down to the new town and the one up to the castle (good restaurants at the top - perfect for lunch, and a walk down afterwards).
The flat itself is wonderfully clean, well-equipped and comfortable, and properly air-conditioned. In addition, despite its central location there is little or no noticeable noise from the town itself - although as others have mentioned the bells are regular, loud, and unavoidable. The flat is on the top floor and there is no lift, but the host did help with carrying our suitcases up when we arrived.
Prior communication with the host was clear throughout and check-in was exactly when we wanted. When we arrived he didn't really speak English, but between us we managed perfectly well in basic French. The No 1 bus journey to and from the airport and station was simple, and the stop was only a few minutes' walk from the flat, with no difficult slopes or steps.
All in all a highly recommended place to stay and a lovely city well worth visiting for a short break.