We had originally planned to stay for about 3/4 nights but ended up leaving after two. It would seem that noise is a problem for this hostel. During our stay, 'Love fest 2015' (not a joke) was taking place one block over and blasted music out from early evening until midnight. It was so loud that it rattled the A/C on the wall in our bedroom - FROM AN ENTIRE BLOCK AWAY - you had to be there to believe it! How people could bear being anywhere near the actual festival is a mystery. If this wasn't enough, everyday, all day from 8am a convoy of 4x4 trucks drove up and down the (only) main road in Mae Sai, next to Afterglow Hostel, blaring out adverts and Westlife music. Now, I realise that these events are nothing to do with the Hostel per se but looking at other reviews, noise does seem to be an issue.
So, if you are looking for a quieter stay i would suggest finding somewhere off the main road, as we did. It's a shame really because the Hostel is decent and you can tell that someone has put a lot of effort into doing it up (if polished, varnished concrete is your thing). They did try to put us in one of the 'cottages' out back but we asked to see another room and the rooms in the main building are much nicer, albeit when there's no noise.
With regards to the staff, they don't seem to speak much English and when we enquired about the times for (free-included) breakfast we were told that they they no longer did breakfasts, only to find a selection of fruit and hot/cold drinks when we came down the next morning that other guests were helping themselves to. Odd. They had what seemed to be a cafe in the lobby but it was permanently closed and looked like it hadn't seen any customers for a while...ever?
Not sure I'd go back. Find somewhere closer to the border and/or off the main road.