Use common sense.

As long as you avoid downtown Johannesburg and Khayelitsha, and some other townships unguided, it is generally safe.
The latest 4 years I traveled to SA in Gauteng+MP+Kruger in 2013 and Western Cape in 2014/5/6.

No safety issues.

You need a cellphone data plan ? Well, get a Vodafone, MTN or CellC simonly. About R300 ($21) with 2GB and 60...120 call minutes (upgradable even at any shop or even gas station) within ZA and both readily available at Vodafone / MTN / CellC shops at JNB or CPT. The employee will install and activate the sim card as well for you.

Most hotels and guesthouses have wifi but that is sometimes capped (you pay per data unit) and sometimes it requires vouchers with a cryptic login code which you have to enter EVERY time you lift the laptop lid or turn on the tablet / smartphone screen.