Recently I stayed a week at the Regnum Residence Hotel. It was my third or fourth stay at this hotel. While many aspects were good, it wasn't up to its previous high standard.
First the good: The location will appeal to many, centrally located on the Buda side, away from the main tourist areas of Pest, but close to the Danube and 10 minutes' walk from the metro. Front desk staff in particular were friendly and helpful. Rooms are a nice size with a safe, small fridge, and kettle with coffee and tea. Wifi was fast and reliable. There is a gym, grocery stores and pharmacies in the area.
However, we had a few disappointments, none major, but overall a bit of a decline from previous visits. As others have noted, the breakfast buffet is a bit lacking. Some items were not hot, and french fries were used a replacement when they ran out of a regular item. The hot sausages, breads and jams could have been of higher quality. Arriving after the main breakfast rush, many tables would be missing knives or forks. Maybe for an included breakfast, it's okay. The housekeeping was a bit irregular. More than once, when leaving my room around noon, I would put the "service" sign on my door, then come back several hours later to find the room had not been serviced. The lack of electrical outlets in the room, specifically near the bed, was a bit unusual. We had to charge our phones on the floor beside the fridge.
Overall, a decent option with strengths, but with room for improvement.