We stayed in Berta's apartment three months and had a great time. We felt home from the very first moment we got there.
The apartment is well located - 15min walking from all the main tourist sights in Seville and by metro (the station is just around the corner) it's only 5min to the city center!
The neighborhood is safe and there are a lot of restaurants, bars, supermarkets, pharmacies and ATM's close by.
The apartment is on the 4th floor and there's no elevator which can be a downside.
It's spacious, bright and clean, beds are comfortable, kitchen is fully equipped and Internet was super fast!
Check in and check out went smooth and Berta was always available and responding within an hour! She had toiletries ready for us as well as a lot of recommendations where to go, eat, what to see etc.
Even though the apartment is really nice I would recommend renting a room rather then the whole apartment (especially if renting for a longer stay). Stove, hot water and heating are based and working on gas bottles which need to be exchanged when empty and I can see this being a problem for some people who are not used to this kind of system. Of course this only occurs if you are renting the whole apartment, if renting just a room she is taking care of it!