Stayed there 2 nights paying really low fare, since I've booked from Accorhotels website and there was a "last minute promotion", less than 150 euros for the two nights.
The staff was a bit rude when we've arrived, as they didn't find my reservation and said that "the system shows me I'm fully booked". But then, the man reloaded his system and there was my reservation.
I didn't pay for the breakfast but I wouldn't recommend it, as long as there're a of choices just crossing the street and walking a little (it seems that there's a college about 300m crossing the street and there're a lot of cafeterias, maybe for the students).
The location is amazing, from the roof you can take nice pictures of the Coliseum, that is just a few steps from the hotel, the "coliseo metro" station as well. About the roof, all complainings about no staff in the roof bar are true. I've been in the roof the two days I've stayed there and I've only seen a guy cleaning the pool.
The room and the bathroom are a little old, needing refubishment, but OK for the price I've payed.
The regular fare is unreal, more than 150 euros per night, and if you don't get a promotion as I got, don't stay there.

I was concerned after reading some of the reviews about this hotel but was relieved and wonder why the bad review after our amazing experience in Rome with this hotel. We did booked a superior double room (and it's a proper double bed which overlooks the colosseum). Our room wasn't big but it was big enough for us (typical European sized hotel) with decent amenities - good toiletries, plenty of towels, cups, LCD tv, hangers, safe box, long mirror and a decent bed with additional pillows. Oh yes - air-conditioning too! We stayed in room 306 and booked through special rates over internet which made it extremely good value for money considering the expanse of hotels in Rome. It's 5 mins walk to the Colosseo metro and it's nice to see the Colosseum in day/evening/night (ok we got a bit obssessed). If not there are bus stops right across the hotel which takes into town directly or at least to Piazza de Venezia which linked to the rest of the place. The room was very quiet considering the main road was outside. There was also the added bonus of a Trattoria/Pizzeria called Luzzi which is across the hotel recommended if you want a proper Italian joint to eat (rather than the semi-pretentious expensive places in centre of town) as a 1 litre of extremely good white wine sets back at €4!!!! The only downside is that this place is so good that waiting time is about 10-20 mins - more so on a Saturday night which is full of Romans on a night out. The hotel staff were very helpful and courteous. We wanted to extend half a day more as our flight was later on that night but was politely told that the hotel was full but we could use the shower facilities/swimming pool and garden terrace if we so wish and keep our luggage in. We loved this hotel and would definitely come back again.. this time perhaps with our swimming kit!!

Our trip did not start out that great since our luggage was not put on the plane we arrived with. We eventually got it the next day.
Now about the Delta Colosseo, I was abit worried when I read the hotel reviews.
However we had a wonderful stay at this hotel. The staff were amazingly great and helpful. The room we got was nice though it needs abit of refurbishing but it was nice. The location was the absolute best and not far away from from the main train station if you take the metro.
We had alovely stay and consider returning.

My wife & i recently returned from a 3 night stay, we had a superior room with a view of the colosseum, the room was small like many Italian hotels, bathroom was small and shower/bath was extremely small, room was clean, some carpet stains.
The a/c was excellent and was needed by us, and a must have for the hot summer months. It was disapointing the rooftop bar was closed, we were told it only opens in june ! Bar service is slow, even when only a few in the bar, staff are very nice.
Hotel is 5 mins walk to the Colosseum and metro stop. Ice cream shop next to the hotel is fab. we did not eat in the hotel with breakfast at €15, there are several places to eat on the way to the colossuem and also to the left of the hotel.
would overall recommend the hotel & location, metro is easy to use to get around Rome, buy a €2 ticket for all day travel & definitely bring comfortable walking shoes.