Derry Villas (also known as Welcome Rest, as the sign at the front of the building says) is a basic guest house / hostel. I stayed here for 2 nights as a stop over, and chose it because it was so cheap.
I have nothing negative to say about this place. Yes it is basic, but that is what you are paying for. The room was clean, comfortable and warm. There were tea and coffee making facilities, clean towels and even a fridge in my room. Free wi fi was available. The communal toilets and showers were spotless. A communal kitchen, dining area and TV room were available and again these were clean and comfortable. Owners/workers friendly.
If you are looking for a budget place to stay in Plymouth, you will not find better than here.