Elena' place in Rome is great. From Roma's Termini it's not too far a walk down the road to get to.
There's a local supermarket about 50 meters away for everything which was really handy and as we found out a load of really great restaurants around the area which not only served amazing food but were really cheap.
You have to check out too the famous local ice cream shop (Palazzo del Freddo Giovanni Fassi) which does around 30 different flavours and is also dirt cheap. Amazing!
As for walking to the colosseum and sites of Rome, it was not that far away and a great way to see the city, but if you are tired the local metro goes to Roma termini or you have the local trams which stop outside the flat.
I would definitely recommend Elena's flat as a place to stay, and with the additional outside terrace, it's a great way to spend the morning before heading out and exploring Rome.