I stayed in Hostel Helvetia for three nights and was happy with it. I had booked a three bed female only room, which was comfortable and clean. I arrived a little earlier than expected ( I had taken an earlier train from Krakow) but was disappointed to find that I couldn't get into the room until 2 o'clock. Someone else was in the room already, so I wasn't sure what the problem was. The hostel was very close to the main tourist area of Warsaw, just a few minute's walk from the Royal Route.
The breakfast was good, with plenty of bread, cheese, ham and cereal. I did think it was quite strange to have to pay for a towel - I thought most good hostels these days provided these free.
The hostel felt very secure - a code, which was changed daily, gave you access through the main door instead of a key.
I was glad I chose the smaller room rather than a large dorm. It was a little more expensive but worth it.