IMac vs HP touch smart 600

My sis just approach me about buying a new desktop. She wanted a all in one pc and I said imac right away. I've been a windows users since I 1st saw a computer when I was 10. ( I'm 24 now btw.) but is iMac really that good? Any iMac user that can advise me? We are going down to the IT show @ suntec city this Friday to have a look.