Hi Stereolab!
I live in Ngaio. Its a very family friendly suburb and 10 mins from town. Option of train or bus to commute if you don't want to drive. Good local kindy, well respected school, local library and small shopping centre, nice playground. Easy drive to 3 supermarkets and more shops in Khandallah including a post office. But on the downside it's not zoned for Wellington College if that's your preference for your DS - only choice for boys is Onslow (co-ed). Have a look at their recent (March 2011) ERO report though. Well Coll is much more traditional boys schools. Depends what will suit your son really. Both are decile 10. My DD1 actually goes to Thorndon School and she loves it there. I drop her on the way into work. She's keen to go to Onslow although she could go to Wellington Girls and most of the girls at Thorndon School probably will go there. Onslow is more welcoming of students that are slightly outside the norm, has a larger subject choice, good Gifted Ed programme etc. Well Coll generally regarded as more academic though and sporty.
Karori is sometimes labelled NZ's largest suburb. Consequently it has a wide range of community facilities. However Karori Normal primary is the biggest primary school in Wellington so you might want to consider that. I know 2 families who chose to drive to the next closest school as they didn't like KN's size. Karori is also a pain to get in and out of, one road gets very congested, and only a bus service (albeit very frequent).
Wilton is good if you want reasonably priced housing not too far from the city and generally quite sunny. However there are no local shops or amenities there at all (other than the school). The closest are in Northland and they're not that fantastic either (loads of takeaways). It is very leafy though, lots of birdlife etc.
The best value suburb in Wellington is Johnsonville but it is much more of a mixed community with quite a bit of HNZC housing. Great local amenities though and easy access to drive north up the coast. Also good train service 20 mins to CBD, more like a tram than a train really. Really nice houses there for under $500k easily.
Wadestown and Kelburn are generally regarded as a bit posh. Both have some cheaper properties but they are often up 50 steps. Both have good primary schools and are zoned for Well Coll. You can walk into the CBD from both and Kelburn has cable car to take you home! I grew up in Kelburn and have fond memories of it. The university gives it a bit of character.
Brooklyn houses often have great harbour views. It also has a nice local cinema and some reasonable shops plus library. School has a reasonable reputation. There are some very windy streets in Brooklyn though - up by the eind turbine for example!
Happy to answer any questions you have. I've lived in Kelburn, Hataitai, Northland, Brooklyn, Ngaio, Thorndon and CBD.