We had a very nice stay at Viminale. The location is great and you can reach a lot of sights by foot (15-20 min). The hosts are super friendly and they made it possible to check in before 14:00pm which was very convenient. The nespresso machine was a life saver in the morning :)
One little note, the accommodation is not for people who want to sleep in (due to the shared kitchen in the hall). We were up early anyway to explore as much from Rome as possible in 3 days. Our room was a bit loud until 12/1am as we were above a restaurant, so pack your earplugs ;)