A good choice, a bit overpriced
We stayed at the Opal in May and as a family with two teenage children we can say that we were fairly happy with our choice. The hotel is in a central area of San Francisco, just a 5 minute walk from Union Square and not to far from the Fishermen Wharf. However, the neighbourhood can be kind of sketchy, lots of homeless people, especially in the early morning. 
The staff was overall polite, but I still cannot believe the shocking amount of money that we were asked to pay for the hotel parking. 34 dollars and change a night! We were staying four nights and with that amount we could have almost afforded a fifth night in San Fran.
The big disappointment was the breakfast. Unless you like grape jelly and whole wheat toast, I wouldn't stick around for the meal. Maybe visit the diner nearby, it looked pretty good. 
The rooms were ok, no microwave nor fridge, and the toilet would stop working occasionally.