Overall, my experience at the Hotel Slavyanska Beseda to be just OK. The hotel is desperately in need of a major refurbishment, but even in its current condition, it remains functional and acceptable for short-term stays. The two biggest positives, in my opinion, are its central location with the city of Sofia and it is a pet friendly hotel (though negatively, they charge extra for pets at 25BGN per night....which i think is a bit expensive).
My assigned room was located on their top floor (accessible by an old, but fully operational, elevator....looks like the original one when the place was first built), and had a large balcony, but the actual room was very small. Like the hotel, my room was in dire need of a refurbishment, and in its current state, looked a bit dirty (only due to wear and tear), especially the single bed. I did complain about the bad sagging, and they sent someone to fix it. Though unsure what (or if anything) was done as there was no improvement. Instead of complaining again, I just used my dog's mattress (from her dog crate) to bolster the middle. The room was only very basically furnished but did have a TV and a small refrigerator (minibar with drinks & snacks), along with complimentary tea & coffee (plus a kettle). Heating in the room was sufficiently achieved mostly by an air-conditioner and very minimally via a radiator heater.
The private bathroom was a little more spacious than the living/sleeping area and was clean and practical as well as functional, with a good constant supply of hot water.
I didn't eat anything from the hotel's restaurant as I found a vast selection of restaurants, all within a short walk plus a small supermarket only 50 metres away (and a Lidl about a 10 minute walk along the main street). The hotel is also situated near to all the main tourist sights/attractions in Sofia, which is very handy and are all easily within walking distances.
I find all the ex-communist countries from the defunct USSR, lacking a bit in hospitality and friendliness, but the staff here were quite polite, friendly, and seemingly helpful. (Reception staff also speak English too).
Noteworthy, if you have your own vehicle, parking on the street and surrounding streets are in very short supply (illegal parking will result in your car being towed away) but the hotel does offer paid parking (spots are very limited and a bit pricey too).