Sight for sore eyes

What does a "sight for sore eyes" mean excatly? I've had people tell me it is a negative saying, but I always thought it was positive, implying something what somehow soothing. Anyone know?

It originally meant something beautiful and delightful, the idea being that beautiful things physically soothed the eyes whereas ugly things made them strained and sore. Of course it can be used sarcastically to mean the opposite.

I suppose almost anything can be said sarcastically, but I have never heard this phrase thus used. I have always heard "a sight for sore eyes" used as an expression of enthusiasm. SS