I recently stayed at J Hotel on two separate occasions. We used this hotel as a transit point in and out of Mongolia, so did not explore Ulaan Bataar in any detail, so cannot comment fully on the location nor proximity to sights.
The room itself was well-sized and clean. The best part was that there was a washing machine, which was very welcome after 1.5 weeks on the train. The beds were comfortable and some basic coffee/tea supplies were provided. It was also possible to order food from the restaurant downstairs and have it delivered to the room as room service. The water pressure for the shower was good.
Customer service could be improved, however. I emailed the hotel in advance to ask them to accept delivery of our train tickets, and there was a long delay before receiving a response which was not understandable (poor English) so we had to make other arrangements. In addition, the front desk was already aware that we had an early departure (around 6am), as we had already informed them off this upon check-in, but at 11:30pm, when I was sleeping, the front desk called to check what time were were leaving (no idea why they needed to call). This was very disruptive and inconsiderate. In addition, I was told on checking in that I could charge the room service to my room and pay the next morning, but the person who delivered the meal insisted on cash payment immediately. Whilst I was able to convince him that I would pay the next morning, he was rather rude about the whole thing. in terms of breakfast, our breakfast was included in our room price and it was mediocre.
Overall, however, the room was decently priced, clean and functional and the use of the washer machine was very helpful. I would give this establishment a rating of 3.5 , but have rounded up to 4.