We stayed at Danielle's loft for almost 2 months. I have previously stayed at over 20 AirBnB places-- I can honestly say that Danielle is one of, if not the best, hosts of an AirBnB rental that I have encountered. She does it as a business and it shows. As random small things came up during our 2 month stay, she responded very quickly, professionally, pleasantly, and in general is was obvious that it was important to her that renters were happy and getting what they paid for. She is an excellent AirBnb host who will go out of her way to make sure your stay is great.
The loft itself is very nice. Spacious, modern, has everything you would need (TV w Cable, full kitchen, furniture, washer/dryer, etc). The building it is in is all lofts, mainly privately owned. The pictures on this site are very accurate of the interior.
The area the loft is in is just on the south end of San Marco's 'nice' retail district. The immediate area is not the nicest part of San Marco, but is safe. Basically directly under 95 which is not the most pleasant view.
As others have alluded to, there are neighbors that can be heard at various times of the day/night (specifically a small dog barks during the day and someone above plays music throughout the night 1-2x per week). One apparently accused us of smoking ( we don't) prompting a phonecall within minutes of our arrival. Otherwise everyone we encountered seemed nice. Parking is right outside the door and was never a problem.
Also the loft is on the street level and has large windows-- Danielle has installed privacy shades that do not allow someone to see inside the unit, but to keep this privacy requires keeping the shade down during the day which keeps the unit pretty dark. Street noise basically goes away after rush hour.
Overall, a good option to be in San Marco for a long-term rental and Danielle will be very responsive to any issue that may arise when you are staying there.