My girlfriend and I stayed at Maria's place for about 2 1/2 weeks.
TLDR: We recommend her place for one/two people who are staying less than 2 weeks and plan to do occasional cooking.
Pros:
Personally we felt that the place is probably perfect for one person or two and for a shorter stay. It's within walking distance of the metro, a lot of restaurants, markets, a few parks, and the old convent which is perfect for seeing Lisbon from the top of one of its hills. It's also not too far from the tourist-district yet far enough away that you get more of the local flavor rather than tourist prices.
Not-so-pros:
Unfortunately, for two people and a longer stay (particularly if you're working), it had some notable drawbacks.
Size: It's a bit hard to gauge the overall size of the place from the photos, but frankly it's tight for two people or anyone who's looking for a longer stay. We are not very big or tall people (both around 5'8'' and 150-170lb), and while it has high ceilings, the closer quarters were starting to get old at times, particularly in the kitchen and bathroom. To be fair, it is not an expensive place and it's in an older part of town. Still, cooking was tough given there is a stone lintel right above the stove, and it's low enough that, unless you're 5'6" or shorter, you're going to have to duck whenever trying to use the stove/counter.
Amenities: Overall, it seemed like the place could have been better stocked given the length of our stay. The cooking essentials were hit-or-miss (good on salt and balsamic vinegar, low on olive oil and black pepper; the knives were dull and needed sharpening and one of the two frying pans was missing a handle, and we were running low on things like trash bags, toilet paper, and soap. Maria was quick to get us some more when we reached out. She is very responsive and does what she can, which is SO NICE when you're in a foreign place. Again, none of these are problems if you're eating out regularly and/or staying for less than 2 weeks.
To end on a high note (for anyone still reading), Maria's instructions for getting to her place and the check-in process were easy. She seems like a host who listens and that she's interested in making the guest's experience a good one. I doubt some of the things mentioned here will be a problem in the future.