We had a great time at Julie&Fabio's apartment. The apartment was clean, had enough space to comfortably fit our stuff, and had everything one would need for a stay. View from the balcony is awesome - you can see entire Citta Alta from there! The neighborhood is quiet, there is a small supermarket close to the apartment. The city center and airport are at a comfortable distance.
Julie was very nice and picked us up from the airport (and drove us back there when we were leaving), shown us around and gave good tips as to where to go and what are the different places we might need. In general, the hosts are really pleasant and made us feel very welcomed.
Bergamo is really nice, quiet, pleasant town. Citta Alta is beautiful and (oh God!) the food is good! There are a lot of other towns that are in close proximity to Bergamo in case if you are staying for longer and want to travel around.
We definitely recommend staying at Julie&Fabio's place (and visiting Bergamo if you're up for slow-paced sightseeing with beautiful buildings, mountains and majestic food).