I just changed mine on Sat. Just pull the bastard upwards with a consistent force, it came out gradually on mine, tried to prise the grommet out but it just cracked and tore, so be careful to get the whole grommet out when you get the valve out. Pliers are a must. Take it easy on the valve and it should be fine.
Only took 10 minutes to do.
I'm sure I read somewhere that these should be changed every 50,000 miles or so.
I bought the whole lot, new clips, rubber hose, gromment, valve was fairly cheap, about £30 all in.