The Marriott Circular Quay is well situated for touring Sydney, and for many business appointments. It is about 150 meters from the Circular Quay train station and ferry docks. The opera house is just another 250 meters beyond the ferries and a lovely botanical garden is also nearby. In the heart of downtown, all major services are available in a short walk: banks, restaurants, etc.

I stayed at the hotel twice about 3 days apart. Each time, my room was attractive on a high floor. One view was of the harbor bridge. The other overlooked the opera house.

The concierge lounge had a decent selection for both breakfast and after-work snacks (which you could make a dinner of if you didn't want to go out). The lounge itself was attractive with spectacular views in all directions. Several people attended to the lounge guests keeping it tidy and the food offerings replenished. It can get crowded at peak times. And lounge access comes at a premium -- unless you are a Marriott Gold or Platinum member.

Hotel location great, service really good, staff all helpful. Enjoyed breakfast, especially the cooked to order pancakes. Only downside was price, a bit too expensive

Stayed here for 4 days on business/personal trip. Location of hotel in Circular Quay is very good. After that the hotel is just average but is extremely expensive. I upgraded to a club room (not on the club floor but access to the club) and it was not worth it for A$100 per night. The club breakfast is OK but not terrific; some warm items like eggs and sausage but otherwise a traditional continental breakfast. The staff tell you to make sure to see 'the view' from the club but there is actually only one small window that looks at the harbour and unless you are sitting at the table in front of it you can't see it.
The gym is very good and accessible to all guests. 3 elipticals, 4 treadmills, 2 bikes, plus dumbbels and cable machines and a few benches plus a few weight machines. The room itself was a bit on the small side and the bathroom was also a bit small but serviceabe. We ate at the hotel restaurant one night; we did not chose the buffet which they push but the food was reasonably good but not special. The buffet itself looked good but not great and had a price of A$50 per person.
I travel on business a lot around the world, and for the price this was among the worst value I have experienced. You are clearly paying for the location more than the quality of the hotel. The staff that I encountered were certainly nice but did not appear to want to go out of their way. Concierge service was adequate but you had to really push them to get them to give you restaurant suggestions that weren't the typical marginal tourist fare near the hotel.

Our stay at the Marriott was wonderful, the service was great, rooms were beautiful and it is located walking distance to most everything: the Sydney Harbor Bridge (what a blast the climb was!), Opera House, train station the Royal Botanical Gardens and The Rocks.
Just one thing to be aware of...the taxi's are usually waiting just around the corner from the hotel, don't tell the valet guys you need a taxi in advance, we had to pay for an extra 25 minutes, because we did some last minute shopping and didn't notice until in route to the airport that the meter was running from the time the taxi arrived at the hotel, not from the time we all got in to leave.