There's a bit of confusion in the reviews here. There are actually two restaurants at this…read morelocation--the "Leão d'Ouro," a traditional a la carte place, and, next door, the "Leão Buffet," an inexpensive and casual all-you-can-eat buffet restaurant. I presume they're both under the same management.
I'm reviewing the buffet side.
My buffet this night was £7.90--including a decent salad bar, a choice of two soups, and a hot buffet (including a modest carvery). Everything I tried was decent, and much of it was very good. Drinks and desserts are extra, but reasonable. The atmosphere was casual and a bit chaotic, but it was certainly an efficient and inexpensive route for supper. I emerged stuffed to the gills in less than an hour, for a shade over £10 including a pint of beer. The staff were understandably a bit harried, but generally perfectly pleasant.
You may have to stand in line for a table, but turnover is reasonably brisk. If you break into groups of two or three, you'll be seated faster. The main rush seems to be 7:30-8:00 pm.
The five stars represents decent and plentiful local fare at a great price. I'd be very surprised if there were a better value in the heart of old Lisbon.