We enjoyed a lot our visit in the Museo Nacional de Arte Antiga. What attracted our attention and urged us to go was the temporary exhibition, well advertised on landscapes and forests in Western Art. We ended up though to like much more the permanent collection (paintings, furnitures, ceramics and sculptures)! I believe that it is worth visiting this museum. Pity the museum shop was under renovation process and it was closed. Exhibits are beautifully displayed and it doesn't feel like one of these museums that they are packed with things without an order. Collection is huge and rich and presented in a great way! The only thing we found awkward was that while we pre-purchased the tickets on a different day than the day of visit (we got joined tickets for visiting both permanent and temporary collections) the day of our visit nobody at the side entrance of the museum asked our tickets! There are two main entrances at the museum and we entered the west one. We've been asked for the tickets only when entering the temporary collection. This was really strange because either by mistake there was no entrance control or permanent collection was free that day and we were just charged the full ticket without being informed the free visiting hours.
I strongly recommend this museum along with the Gulbenkian one! You will have no regrets for visiting!