Tana has had many great reviews, so I don't feel that I need to add much, accept that I agree with all the positive things other people have said! I was lucky enough to stay for a month and I loved every minute...
Tana and his business partner Eva are both very friendly and knowledgeable about the City. If there was anything that I needed, all I had to do was ask. I was treated more like a friend than a guest.
The apartment is in excellent condition and has all the facilities you need. Tana even had the place professionally cleaned for me on a weekly basis which was a pleasant surprise. The high-speed broadband is a real bonus, if like me, you use the internet frequently - I was able to stream HD movies without any trouble.
The apartment is in a great location - as long as you don't expect a beach view ;-). The neighbourhood has a local feel which is exactly what I wanted, but there is also plenty of nightlife within walking distance. The City is well served by public transport so it's easy to get around. There is also an extensive network of cycle lanes and it's super easy to rent a bike (take a look at www.valenbisi.es).
It's worth mentioning that in August many of the locals go on their own vacations outside of the City. This meant that some of the local businesses in the immediate vicinity were closed during my stay. However, there are still more than enough places close by for all of your food/drink needs.
Valencia is a great city to visit. It has an interesting mix of the historic and the modern and there are countless things to keep you occupied, whatever your interests. There is a reasonable amount of tourism, more so at the obvious attractions, but the city doesn't feel over ran by it. It's very easy to avoid the crowds and meet the locals if that's what you want. In general I found the Spanish to be very friendly and lots of fun to be around.