Khao Lak's weather is very similar to Phuket's weather: The [relatively] cool dry season lasts from December to March, with rains beginning in April, and becoming heavy by September. The main difference is that it does seem to rain slightly more in Khao Lak. Indeed, in my last stay there over a New Year holiday, it rained every afternoon.
As with the rest of Thailand, the rainy season is not necessarily a bad time to visit. It almost never rains all day. In fact, you might have a quite sunny morning followed by gathering clouds around noon, with rain coming late in the afternoon for an hour so. The rain showers usually come at the perfect time to have a shower and start thinking about where to go for dinner.
You'll also find the "low season" rates at many hotels and resorts are up to 40% less that the "high season" rates.
Worth noting: The rainy season affects different parts of Thailand at slightly different times of year. This is particularly true when it comes to the different sides of the Malay peninsula. At the height of the rainy season on the Andaman side around Khao Lak, it will still be rather dry on the Gulf side, where Samui is located. When planning your trip, you may want to check out our handy weather comparison chart.