I stayed one night and had a mixed experience.
Pros: location is very convenient to the Gautrain. The hotel is generally as expected for a Holiday Inn. Rooms are a little tired but relatively clean.
Cons: the shower design is weird and it's difficult to keep water from spilling out onto the floor - you have to be quick or it overflows. The big issue though is the one mentioned in the title... I checked in around 3pm and put up the Do Not Disturb sign, locked the deadbolt, and also set the extra security latch out of habit. About an hour later someone used a hotel keycard to open the door. The keycard bypassed the deadbolt so the door partially opened -- only the extra latch held it. They didn't knock first and didn't say anything when the latch stopped them, and by the time I looked through the peephole they were gone. The DND sign was still up. I wanted to think it was just a mistake somehow, but given the situation (notoriously unsafe city, ignoring the DND sign, no knock, no apology, using a hotel key...) I didn't feel safe for the rest of my stay, and didn't leave my room until I checked out. I also didn't feel comfortable calling the front desk about it since the person had a hotel key.
Due to that experience I don't feel I can recommend this hotel and I would not stay again.