We had a wonderful stay at Sonder Testaccio. We took a cab from the airport but the bus and metro stops are nearby and we used those too.
The hotel is very nice with 24 hour coffee and water and a lovely "continental" breakfast (for an extra 15 euros) which is really a full buffet. The bed was comfortable, the shower was excellent, and there was a medicine cabinet and drawers in the bathroom which was super helpful and convenient. There is someone at the desk 24 hours and they were all so nice and friendly and ready to help with restaurant recommendations, calling a cab, and anything else that we needed. The Pyramide metro stop is a 10 minute walk and then only two stops to Coliseo. We found the metro very clean, fast, and safe. There is a very cool market, "Mercato di Testaccio" right next door and a contemporary art exhibit called Mattatoio (don't confuse them like we did) on the other side. And we can recommend the restaurant Ai Cocci which had such wonderful service and food.