Like all words based on race, there will be some that take offense.
It is not a derogatory term but I've heard it used in a derogatory way a couple of times. I'd say probably the same as half caste 40 years ago in the UK.
Same as the word 'kaek' for Indians, when used with an ultra low tone, it is derogatory.
50 years ago, the word jek was used for Chinese people, but it has become offensive. I guess it is up to the people involved to say if it is offensive or not. Similar to the 'negro' in the USA. I've used 'jek' to some Chinese/Thai people as they are often the most racist to farang here.