The apartment is exactly as described. The kitchen and living space is plenty big enough for a group of 4 staying and the facilities provided (including the kitchen equipment) would make it easy to cook for yourself if you chose to.
Marcel and Marta's communication before and during our stay was excellent and although we did not meet them personally, we had no problem with checking in and out, and they were very responsive to our questions.
The apartment is situated about 3 minutes walk from Fontana metro station on the green line. The road itself is quite busy at times though we were not overly disturbed by this (apart from some beeping horns early one morning). Connections in to La Rambla were easy by metro - or by a 30 minute walk if you wanted to stretch your legs. Parc Guell is also within walking distance or a short metro/bus ride. The immediate area is not very touristy, but there are plenty of places to buy food (a couple of small supermarkets nearby which had everything we needed), and restaurants and more touristy cafes are a short metro ride away.
The apartment is extremely clean and tidy, and we had a very comfortable stay. The beds are comfortable, and the bedding is good quality. The bathroom and shower is small but worked well.
The bedrooms are quite small, and the twin room does not have any external windows which may be a concern for some people.
The small outside terrace is a nice place to sit for a meal or drink - though it does overlook the side street.
Marcel and Marta provided extremely clear instructions about our stay, and were very helpful allowing us to check in a little earlier than normal as our flight arrived quite early on our first day. In the apartment itself there were just enough instructions to help you to use the facilities easily and a selection of 3 guidebooks (1 in English, 1 in German and 1 in French) were a helpful addition.
Barcelona is a lovely city to visit and we would thoroughly recommend Marta and Marcel's apartment, and would happily stay again.