I've eaten here just now, and once a couple of years ago.
I'll preface this review by saying I don't dine at chinese restaurants very often due to a myriad of poor service and cleanliness experiences at most of them.
So on my first visit to Yum Yum I was very pleasantly surprised by the cleanliness and friendliness.
The food I've had so far has been good enough. Nothing mind blowing, but that's probably just me not finding the interesting items on the menu yet as I'm not a chinese food expert.
Service, I am conflicted about. The staff are friendly, especially what I am guessing are the young owners. They don't seem jaded and seem to be putting a lot into the restaurant. That alone means a lot to me as most chinese places I visit make you feel like you are inconveniencing them with your visit.
Service on the first visit was great. Service on the second visit started out well but deteriorated as the evening went on. They present the dishes in a friendly way, pour your pop, your soup, place the dishes right in front of you, etc. My drink ran out and nobody came by for a while to check. We started dining a bit after 8 and were done by about 9:30 when the restaurant was winding down to close. While a number of people were still eating service seemed to grind to a halt as the staff all seemed more interested in counting up their tips for the night. After about 30 minutes with no bill in sight had to go up front to pay. You could see the other diners in the place wondering what was happening too. My advice is don't come near closing time. I think we were also overcharged but by that time just wanted to get out of there.
Ambience - the room is an open square but the decor is nice and it's clean. I'd give it 4 stars but have to take one off because it was way too hot in there. This was on a cool 20C summer day too. I'm not sure if they have A/C, if not, this place would get ridiculous on a hot day.