Valdemara residence is a student dorm that doubles as a hostel. I only spent one night there as I was passing through Riga, and found it very clean, comfortable and in a great location (not exactly in the center but close to it and with several shops, restaurants and bars 2min away).
My only complaint/suggestion would be to give the door code required to access the building via email before guests arrive, as I after 7pm there is no other way to enter the building except calling a local Latvian number. Since I didn't have roaming in my phone I had to ask some guy in the street to call for me be able to get the code from the receptionist.
Also, if you get one of the rooms facing the main street you might need earplugs as it's quite noisy. But hey, it's a hostel and you can't beat the price. Definitely the best value in Riga, so I'd stay there again for sure when in town.