In my opinion they are quite accurate when there is a solid base and crap vis - also not bad when the wx is pretty good for wind and QNH....... it is the in between when they are not so good when there are multiple layers and fluctuating vis. Like the others said - understand the limitations and work around them - the reason for the fluctuations in things like cloud base/vis between the METARs and TAFs is because one is issued every 30 mins and one every 6 hours or so