I was there, man, almost exactly 4 years ago. They are beautiful, and the air, despite being underground, is pure.
I have asthma (and I'm fat), so a long climb down the stairs would've killed me (not literally), but I was amazed at how good I felt afterwards. It is amazing down there! There's even a long staircase and hall which seems like an unearthed lost kingdom.
And since the air is pure down there (due to the salt), they have rehabilitation clinics for people with bad breathing.