You will find a better place for the same price
We stayed in the Marina Inn for two nights in January 2012. We initially intended to stay for a third night but we did not have a great experience here and we moved on. 
The positives: 
1. The location is good, it's about a half hour walk to downtown. San Francisco is easy to navigate as it is laid out in a grid. 
2. The room was very big and bright. It was clean and the bed was comfy
The negatives: 
1. It was very noisy. On the first evening we could hear the tv in the room above us nearly word for word. This wouldn't have bothered us too much because they turned the tv off about 10pm or 11pm so we weren't kept awake by it. However, about midnight the couple in the room next to us started... I think they were drinking in their room, I'm not sure, anyway it was VERY noisy. We lay awake until about 1am hoping that it would stop. Then I went and knocked on the door of the room and asked them to keep it down. There was no improvement and so we rang reception and asked if they could either have a polite word with the room next door or if they could put us in a different room. Reception was very rude and treated us like we were making the whole thing up. The room next door finally went to sleep about 5am but our sight seeing plans were really impaired by our lack of sleep. 
2. Parking
There is no parking with the hotel. We arrived on a Sunday and so the receptionist told us to park on the street. She did not suggest that we park in the multi storey car park down the road and she did not mention any risk of towing. Upon our return from sight seeing our car was gone from the street. The receptionist (the same one who told us to park on the street) said "oh you shouldn't have parked on the street, it's a really bad area for towing". We had to pay $382 to get our car back and we then had to pay a $90 parking ticket. 
The room was oldfashioned and we could have found somewhere nicer with parking for the same money.