We enjoyed our stay at Mario's place and he was very accommodating and helpful. The apartment is a nice size and it's quiet with nice views and decently equipped. Mario gave us a lot of good pointers on places to eat and things to see that helped us feel like locals.
The area it's in is lovely in the sense that it's the highest point in Lisbon so it has great views but if you're walking you can plan on walking up or down that hill. We didn't mind the exercise but maybe that's not for everyone.
It felt totally safe and we would have no problem recommending it to friends.