I have frequently been warned about this place and have avoided it until my most recent trip when I decided to walk up. I spoke to several locals about the level of risk and decided to take a minimal amount of money (just so I had something to give potential thieves), water, and my phone.
There weren't many other people around, and I walked the 7-8 minutes up to the cross alone with absolutely no problem. I saw a few people coming down (a man with a small child, a Guatemalan couple, and an older lady). Once I got up there, I met a few fellow tourists as well as Guatemalan families that had set up small stalls on the hill with standard tourist items for sale.
The view of the city and of Volcan Agua is stunning, especially on a sunny day.
Be aware of the risk, don't carry valuables, and definitely don't go at night, but it's a gorgeous view of the city!