I was at the Westin for 5 days in January. We arrived by car from Stellenbosch after 2 days there (and had the car for more than a week driving from Port Elizabeth). When we arrived I dropped my family off and returned the car to Hertz which was a 10 minute walk (safe during daylight).

We did fine without the car for a couple days, but did end renting a car for 2 of our days there (this time from Avis - same 10 minute walk). We were able to get to the V&A Waterfront several times using the hotel shuttle (and walked back once, again during daylight). So first day was mainly spent down there at the aquarium and shopping. And second day we saw most of the main city sites on the Hop On/Hop Off bus mentioned above - which was great. Was not missing a car.

But after that we wanted to drive around the Cape including visiting the penguins in Boulder's Beach and driving Chapman's Peak and it just seemed easiest to have our own car. Plus on the day we did the bus, Table Mountain was closed - so having the car to go there first thing in the next morning vs. having to buy another pass or hire a taxi (especially if getting the car anyways) fit our needs. Then on the last day we used the car to go back to Camps Bay for beach time and then head out to World of Birds and Kirstenbosch Gardens. This would have been doable on the blue bus line, but having the car let us go the opposite direction and be on our own time.

For ease would have preferred to use the Europecar in the hotel, but could not get anywhere close to my corporate rate at Avis so made the walk up the hill. In general I think you can do fine without a car, but I found driving around Cape Town quite easy and having one for a few days can enhance your experience as well.