I can't speak for everyone's stay at this hotel, but we had a great experience.
We booked three nights at Albergo Quirinale via Gate1 (bundled with airfare, breakfast buffet included) during a wet December (b/t Xmas and New Year). The hotel isn't the cheapest, but it is convenient, spacious and pretty, and the staff is nice and helpful (English-speaking, friendly and knowledgeable). Many of the amenities are, in fact, superb.
There seems to be a lot of discussion about the "four star/three star" status of Quirinale on tripadvisor reviews here. Look--it's not a budget hotel, but few hotels in Rome are; however, if you get the unlimited breakfast buffett, you can stuff yourself with food and really work the location so you save and see everything (it's hard to get a better place of operations, esp with the ROMA Pass rail/bus savings). And you'll enjoy your time there.
Do some research, though: Piazza della Repubblica is about 2 minutes by foot, so you can foot it to buses (the 910 for the Galleria Borghese), the 40 Express to the Vatican (stops two feet in front of the hotel, opposite side of Via Nazionale) and subway. The Baths of Diocletian and the National Museum can be walked to as well in about 5 mins. The main post office is three blocks away; ATMs/banks are plentiful on both sides of the street; Roma Termini is 10 minutes by foot to the south; Santa Maria della Vittoria, with Bernini's Ecstasy of St. Teresa (and other churches, including San Carlino delle Q.Fontane--all free) are about 10 minutes away. The Trevi is walkable, too, along with the Palazzo Barberini. ... If you need them, there are also magazine stands and vendors close by (but the hotel has English/international newspapers in lobby), and a small, charming supermarket down the road on the opposite side of street will furnish you with the rest of your needs.
In terms of the hotel itself, it adjoins the Opera House (Verdi stayed there), and staff can tell you how to book tix for performances. You can expect the sculpture courtyard to be full of people in warm weather, and the Sala Verdi (where the buffet is served) to be packed in high season, but that's typical. Ditto for the restaurant (mid-range in terms of price, but delicious) and the bar. There's a postal drop by the desk for international/domestic mail, and the elevator works (the bathrooms by the lobby are also clean, and may also be handy for you--Italy's WC situation is, travel-wise, something to be aware of). It was still holiday-festive inside when we were there (Christmas tree, warm fire, music in the evenings), so summer may be very different in terms of atmosphere, etc.
We were on the 5th Floor, and it was quiet, but there was some noise from a nearby room with a family for an hour or so (always bring earplugs on holiday). Room had a safe, very good windows, and a double door, and the beds (two twins pushed together, per Italian custom) had marquetry with pastoral scenes, as did the cabinets. TV was great (flatscreen, many channels), and the bathroom/shower worked perfectly well and was stocked and spacious (minibar ditto). Ask for an interior room ("una camera tranquilla, interiore") so you don't get one overlooking the noisy Via Nazionale. As with all services in Italy, a little Italian goes a long way in terms of help and friendliness.
Note: in some reviews, you will hear that a hotel's "AC shut off for the afternoon" or that "the heat didn't work for five hours!!!" That's not the hotel's fault: that's Italy--by national law as an energy-saving measure, they shut off the heat/AC at times to save on power. A bit of research might alleviate this concern; so maybe if they bought a Rick Steves guide they'd save on stress before the trip, instead of spending more energy and time worrying when in Italy?
Anyway, the Hotel Quirinale isn't the Four Seasons, but it has charm and is very convenient. We would definitely stay there again in a heartbeat.