I HIGHLY recommend Manuel's place.
It is very conveniently located (very close to Termini station), and walking distance from basically most highlights in Roma - well... I do really walk a lot, so this may not be true for just anyone. But the Cavour subway stop is very close to the apartment.
The neighborhood is super nice and you can get basically anything you may need (restaurants, supermarkets, nightlife).
Manuel is, indeed, a great host: super friendly, very easy communication, very helpful - in case of need, you can rely on him.
Rome... well, I don't think it needs any endorsement! ;)
ENJOY ROME AT MANUEL's PLACE!!!