Northpark is a slum, mostly. I know I live there. Its main attraction is that its cheaper than Hillcrest but not as crappy as city heights. My neighbors and I all go into Hillcrest to do our shopping and dining.
There are some very nice houses in the morely field area and a few on the rim of Mission Valley. That is a tiny percentage though. The only reason I can think that a house would be priced at 600k in NP, is because the seller thinks he can get it.