This is basically a stay at a hotel right in the heart of town, a very good value. You're right in Union Square - tons of restaurants and shopping, good night life, nice view of the city.
Only thing to be aware of relates not just to the Airbnb, but every hotel in the area. You are right in Union Square. That means in addition to the perks, you have the noise and the cable car bells and the crowds etc. Not necessarily a bad thing if you came to experience SF, but something to be keep in mind.