This review is from our own experience, I should point out that it didn't spoil our stay in Rome at all, but it wasn't ideal...!
The entrance lobby, bar, restuarant and public ammenities were all very well presented, modern, clean and welcoming. The staff were polite, professional, welcoming and helpful.
However, the room was not what we would expect from a 4 star hotel.
We were on the 6th floor - leaving the lift you instantly notice the dated decor and shabby carpet.
Once in our room we both agreed it was more like a basic 3 star (Ibis type room) - the curtains were hanging off the rails at points, the double bed was actually two singles pushed together and the bathroom was tiny - all of that we could have accepted without comment had it not been for the dreadfully cold room..
We fiddled with the heating for the first day thinking it needed time to heat up and when we got back late it was still cold and the air was coming out luke-warm.
We checked the windows and they clearly we not very well fitted as there was cold air coming in around the seals (which also meant it was not well sound-proofed). So decided to turn it off all-together and try again the next day.
We were up and out early, so turned the heat thermostat up full blast, got back late and once again it was cold. We were shattered so we went to bed in all our clothes!
The following day after a dreadful nights sleep from the cold and the noise outside (due to limited sound-proofing) we spoke with reception.
They agreed to send a maintenance person in to check but pointed out also that they only turn the heating on a few hours at the start and end of day (not what we expect from a 4* hotel).
When we returned the room was no warmer, all that was left was a rag on the side (maybe from the engineer) so we had to ask (twice) for a blanket for the bed and in the end my husband collected it himself.
As we were by this stage leaving the following day we didn't want the hassle of moving room and wished we had said something on arrival (but were too excited to stop as wanted to see the sights) so only noticed when we went to go to bed that first evening... Plus surely it is the hotel's responsibility to check these things so we don't have to anyway? It is clearly a recognised issue based upon the other feedback on here?
It's such a shame as the staff were so nice, but we would probably never stay in a Mercure hotel again and sadly not in the Roma Delta Colosseo - tatty round the edges we can put-up with to a limit, but not cold and noisy!

The hotel location is quite convenient - located close to the collesium and Forum. After a long day of travel I really appreciated being able to checkin early.

The single hotel room was small but perfectly adequate. My room had a view of the collesium which was great. The hotel staff were very helpful and friendly.

We stayed for 3 nights in this hotel. We arrived a few hours before check-in but the room was ready and the check-in guy that served us friendly and efficient. The room was a bit unusual in that it had a big pillar in the room and a porthole shaped window on one side. The bed was comfortable – not too hard. The air-conditioning in the room worked well which was good as it was fairly warm when we were there. The room had a fridge which also worked well. The shower was in the bathtub. The balcony was shared with the room next door & was enclosed in glass which made it quite warm but did keep out a lot of the traffic noise. From the balcony there was a view of the Colosseum which was just down the road.