I have heard a lot of horror stories about accommodation in Rome which can be quite small and dirty, while also expensive. However, the Marco Polo B&B is extremely clean while we had a spacious double room as well. It is also not as expensive since it is not situated in the busy Rome centre. It is walking distance from the Vatican City which is great and there are a few nice and cheaper restaurants nearby. The metro is not far so it is not a problem to get to the centre of Rome fast. The only downsides for us were the fact that there were no carpets in the rooms so it remained kind of cold, but since you are out and about most of the time it is not really a problem. The breakfast is also not great ... everything is very sweet (although we found that this was an Italy thing) and there is no kettle. So if you want boiled water, you have to use the stove. The staff are however very helpful. If we go to Rome again, I would definitely book this B&B again ... it is definitely value for money!
