I had a great time staying at Juliette! The hostel is located only a 20-minute walk from the edge of Viejo San Juan, a block or two from the Capitol (and its little adjacent beach), and across the street from an excellent little outdoor café (Las Puertas) that serves empanadillas late into the evening. The staff are helpful and friendly, and I enjoyed drinking the provided free tea while working in the air-conditioned check-in area in the evenings (although the bedrooms are equally air-conditioned and also provide strong WiFi access). The beds are spacious (two pillows wide!) and come with built-in hangings, plus adjacent lockers with locks provided (or you can use your own).
My biggest issue with the amenities provided by the hostel was the very cold temperature maintained in the bedrooms. Blankets are not provided with the bed linens, and I was not kept very warm by the blanket that I was able to rent from the front desk (a flat fee of $12 for my entire stay that also included a rented towel and shampoo/conditioner/soap). Anyone who similarly runs on the colder side might consider bringing a heavier coat to layer on top of their blanket. (The hostel also did not have running water for part of my stay, but this definitely was not their fault, as San Juan was facing a major water shortage and water was being brought in by truck for even the fanciest hotels in Viejo San Juan.)