I've never had a problem with Air Asia.
But, when I had to get from Koh Phangan to KL, overland was significantly cheaper. Train the whole way would have been great, but it was a last minute decision and the train was booked (get tickets in advance if you wanna train it). I took the bus. For me that meant...
Taxi Thong Nai Pan to Thongsala. Overnight Thongsala.
Early boat from Koh Phangan to mainland.
Bus to bus stop outside Surathani. Bus to Butterworth. Arrive 8pmish. Overnight butterworth.
Morning train to KL. Arrive early afternoon.
Long trip but not terrible. You should have a good reason to skip everything in between though...