Well, it is definitely not a place to stay for 2-3 nights. For one day excursion, it is ok.
Sunway is about 30 mins away from the city centre if you travel by taxi. Except the park, there is a shopping mall. The surrounding is local residential area. I would perfer to stay in the city centre.
I went there with my children around three years ago. It is quite a big theme park, but it is crowded (especially on public holiday). It is because the park is opened for public. I also agree with 肥仔媽咪 that the facilities are old and the cleaningness is not very good.