I have watched on many occasions Alderney go from CAVOK to worrying about RVR in 10 minutes. Usually the forecast is reliable, but not always.
Due to it's position on the hill no doubt. I was there a couple of weeks ago on a CAVOK forecast day and saw it fall from 9999 to RVR 600m and back again in the space of 20 mins.
Make sure you're prepared, perhaps even take an IMC/IR qualified person with you the first time as there is no doubt that crossing the sea to a new airport is one of the most exciting and valuable things you can do with your licence. Most mainland jollies can be done in the car in a similar amount of time and leave you wondering as you eat your £200 hamburger whether it was all worth it. Turning onto finals at Alderney, Jersey, Guernsey, Le Touquet etc reassure you it's money well spent!