I've never been there but, as you know, I'm at the end of my African Journey planning so I looked for a lot of information about everywhere there.
It seems that one or twoo weeks are enough.
The highlights are specially hiking Mkhaya Reserve to see lot of white and black rhinos and the Mlilwane wildlife sanctuary to see lions and other animals.
White water rafting is also doable on the great Usutu River.
The most important area seems to be the Ezulwini Valley.
Mbabane (Swaziland Capital city) is conected by bus easily with Maputo and Johanesbourgh.
When r u going to Africa? In reference to the title of your post (Swaziland-worth a visit?) and to your own personal homepage, what country Isn't worth a visit?? I think that every country is worth a visit even if only for a day :O)
All I know is that no Visas are required for Europeans to enter Swaziland.
No idea about car rentals but It would be easy in Jburg and get into Swazi. to move around, that's easy attending to what I have readed about the country.
A week seems to be enough.
If you have time, why not? maybe it will worth a visit but only trying you will get the answer.