It's not as nice as Hampstead, but it's way better than Finchley Road or Kilburn - lots of little cafes, good restaurants, cocktail bars etc. If I were to compare it to another area I'd say it's like a slightly quieter, more independent (i.e. less chains) version of Angel.
Several of the roads e.g. Crediton Hill, Honeybourne Road, parts of Lymington Road are all conservation areas. What you need to watch out for are the properties lambled as "West Hampstead" but actually they're Kilburn or worse again... Cricklewood.
Heading south from WH (down West End lane) you're into Abbey Road/St John's Wood - it's slightly less upmarket here, but even the ex-local authority stuff is still well kept.
Disclaimer: Lived in/around West Hamstead for 2 years and adored it. Have never even seen a policy yellow notice board!
Shame I can't afford to buy there...