Ah Italian hotels...I don't know what rating system they use, but this is definitely not what most people would call a 4-star hotel. It is basically a big boxy hotel designed for tour groups. To be fair, I have stayed in much worse and for its purpose, I think it is fine. However, if you are the average tourist on your own, I would not choose to stay here.
First of all, rooms are tiny - however they DO have air conditioning, which is great. Bathrooms were also small and the provided "towels" were more like dishcloths. The tv screen was smaller than my laptop with only CNN (in black and white) for non-Italian speakers to watch. It appeared that some rooms had renovations and used key cards, while others were older and were still using keys, but I don't know if there were any other differences. Another big complaint - the walls were paper thin. I was staying with colleagues who were in the room next door and we could literally have conversations through the walls at normal speaking volume. The breakfast was blah..just dried up croissants, water, stale cereal and juice.
As for access to city center Rome without a tour bus, you can either walk to the subway in about 20 minutes along a busy road or take taxis. It was about 13 euros by Taxi from Piazza Navona, which isn't too bad if you are sharing.
If you are traveling on your own, stay somewhere else. If you are taking a tour and are being put up here, it's not going to be luxury, but it's not terrible and at least you aren't stuck an hour outside of Rome.

There was a group of 51 of us - we had a great time. The hotel isn't central but it only took minutes to get anywhere by taxi and the metro is close. Breakfast could do with a bit of a revamp and unfortunately the pool was closed, but the rooms were comfortable and clean, the staff were helpful, the bar was well stocked! We would recommend this hotel to anyone, its difficult to get a hotel in the centre of Rome that doesn't cost mega money! The Metro in Rome is so easy to use this hotel is great!

Comfy beds, clean rooms, towels changed daily. Like most Italian establishments staff are cool towards guests but we found the front desk helpful enough.

Didn't have a main meal there but breakfast is rather bland

Good Chinese restaurant and several fine pasta establishments/ Pizzerias nearby.

20 mins walk to a good supermarket for water/fruit, and 1 euro bus stop/metro stop to the centre of Rome, (journey approx 20 mins). 8 euros by taxi to Ponte Vittorio Emanuelle. No problem if you don't mind a stroll and riding buses.

I stayed at the Princess with my wife and I have never been more disappointed by a hotel in my life: the hotel is very far from the city center and the nearest underground stop is more than a mile away. You need to walk along a shabby and noisy road to get there because there are very few buses and you'll drop dead from breathing in all the pollution from the bustling street waiting for the bus to arrive!
the hotel is located near the Hilton hotel and other very nice 4 or 5 star hotels but the Princess is certainly not one of them and should never have been awared 4 stars. How deceiving!: our room was small and barely clean, the bathroom tiny and inconvenient and the staff at the reception barely polite!
We have returned to Rome since that dreadful stay at the Princess and have picked a really nice place near the Spanish steps; so my advice to you is: go to Rome but pick your hotel wisely!