Meaning: someone or something you are happy to see 人见人爱、赏心悦目的人或事物；救星
Example:
Man, you're a sight for sore eyes. I've been stuck without a car for two days. Can I borrow yours for a while? (伙计，你可真是我的救星啊。我已经两天没有车开了。你能把你的车借我一会儿吗？)
Use:
This is colloquial American English. If you're in trouble or having problems, and someone who can help you out shows up out of the blue or just in time, you can call him or her "a sight for sore eyes". Or, if you suddenly run into a friend you haven't seen for a long time and you miss him or her, you can also use the idiom, although this is less common.