I live in Knocklyon and love it.
We have been there 10 years this year, had our car stolen twice about 8 years ago (about 2 months apart). Our next door neighbour was burgled one evening about 5 years ago, a guy hanging around saw the back door open and popped his head in, got her bag and car keys. I do feel those sort of things could happen anywhere though.
We live in a fairly quiet cul-de-sac so have no problems with teenagers hanging around. It's a mature estate but there are young families also. Since we moved in there have been less than 5 of our type of house (3 bed) up for sale so I feel that's a good sign.
The shopping centre is just up the road, schools, post office, church etc.
The downside is the transport system, it's my one and only bugbear, the bus service (No.15) is atrocious, there simply aren't enough buses to cope with the amount of people and very little bus lane on the route into town.