Johannesburg and the highveld is warm by day, cold at night, and dry. Cape Town and the Western Cape is warm by day, cool at night, and might be wet and windy with winter rainfall. The Kruger Park and surrounds is hot by day, cool at night, and dry.
May is an ideal time for beach holidays on the Indian Ocean coastline of KwaZulu-Natal coast, which tends to be temperate to hot over the nominal winter months, and very dry. Conditions on the coast of the Eastern and Western Cape are less predictable. Game viewing in the Kruger Park and other reserves tends to improve following the end of the rains, as animals congregate close to perennial water sources and the undergrowth clears to improve visibility.
Accommodation in tourist areas is usually quiet in May. Taking everything into account, May is one of the best months to visit South Africa, particularly if your main interest is safaris, rather than beaches.