We poured a mug of tea into the back of the Macbook Air, it didn't like it. I took it to my local authorized repair shop; they charged me $80 to tell me that the logic board was dead, but that the moisture sensor was tripped so there was nothing they could do about it, they recommended a new laptop as a replacement logic board would run to around $900. I found a refurbished logic board online for $400, bought the necessary pentalobe screwdriver from ifixit and had a go at fixing it myself.
Once I'd surrounded myself with the necessary tools and got down to business the replacement took about an hour and a half. Having no prior experience of cleaning thermal paste residue from a processor I admit that I found instructions containing the phrase "wait for the processor to dry before proceeding" more than a little disconcerting, but it worked out fine. Having to completely disassemble the thing and its many tiny connectors I was convinced that I would have to open it up again a few times to reseat things I hadn't correctly reattached, but it worked perfectly the first time.
The logic board replacement guide told me everything I needed to know and the supplied tools got the job done. I have no complaints.