Summer can be terribly windy (and often stormy in Cape Town). We were there last year (2012-13) and had planned to take the cable car up to the mountain shortly before midnight and celebrate on the top... We got as far as driving to the bottom station of the cable car, where we parked our car for 30 seconds before deciding it was too dangerous (the wind was literally shaking the car back and forth) and drove to Signal Hill instead. Halfway up Signal Hill, the park rangers told us to turn around and that they were closing the road. Fireworks over the harbour (which we did see from the road) were spectacular...

Overall, it can be great, but the weather can be good or bad. Anyway, if you are there for NYE, you must stay for January 2, which is the Cape-Coloured Minstrel Carnival...