I thoroughly recommend Mingle and would definitely stay there again if I come back to KL.
The best thing about staying here is the friendly, helpful and enthusiastic staff. They know the city very well and offer advice on activities, local food, onward travel options, etc. They also organise a group activity each weekend - today they took us for a tour of local restaurants to try some traditional foods.
The dorms have everything I would expect from a new hostel: great wi-fi; beds are really comfortable and each has its own light, air-con unit and power outlet, as well as a thick curtain for a bit of privacy in the dorm; bathrooms are clean and the showers are powerful. There is also an open air seating area upstairs where breakfast is served in the morning.
It also has loads of character and is a quiet, relaxed place in a great location close to lots of things to do. The Chinatown night market is right outside the door so lots of cheap, tasty food is easy to find, as are other things you're likely to need, like ATMs, grocery shops, and a laundromat.
The only negatives I can think of are very small: it would be nice to have a water filter to refill bottles from and stop so many being wasted each day; some rooms don't have windows so in spite of the air-con it can get a bit stuffy if a few people are moving about packing bags... but if you're in the room enough that this bothers you, get out and explore!