The courses are variable. Some are very good, some good, some OK and some just downright poor, so it makes a lot of difference depending on which subject you are thinking of applying for. You need to research how it rates for your particular subject.
The atmosphere is very different depending on the subject and the campus so you may get some conflicting comments.
The reason that it doesn't have a great university atmosphere is because a lot of students are Londoners and live at home and commute to uni and a lot of them have part time jobs at weekends. This means that there is not so much going on at weekends. Another downside, from my observations, is that it tends to be quite socially split - in that that there are quite a lot of mature students and the different ethnic groups don't seem to mix that much.
Having said that people find their own friends and can have a great time.
The main drawback of the town is that rents are expensive. London and the South East is an expensive area. It's not as bad as London but a lot more expensive than Birminham or the North of England. The town has a great shopping centre and good nightlife for a medium sized university town. Nights out in London are perfectly possible if you don't mind getting back in the early hours. London is half an hour on the train but the night buses aren't very frequent and slow getting back.
Ethnically Kingston uni is very diverse but again the social and ethnic mix is very different from one subject to another and from one campus to another. A lot of Asian students from West London like to come go Kingston because they can easily live at home and commute.
The atmosphere on a design course at Knights Park is totally different from that of an IT course at Penrhyn Road. So there aren't any simple answers.