"Is Wi-Fi still a good technology for IoT? Is this cheap and power efficient enough?"
It depends on which Things (the 'T' of IoT) that you are talking about. Bluetooth 4.0 is probably the best for things near you, such as pulse monitors for exercise logging.
For things around the house, the advantage of WiFi is that it exists in almost everyone's house. If you created a WiFi controlled plug or garden sprinkler system, for example, anyone could buy one, plug it in and immediately use it.
Some other RF network, such as Zigbee may be smaller and lower power, but it requires a separate controller.
For objects used out of the house (such as a tablet), what other choice is there but WiFi?
I agree with tb1. If it's "personal area" you're talking about, something like Bluetooth might make more sense. But much of IoT is not just personal area. It's at least room area, if not whole house area, which is exactly what WiFi is designed for.
So then the question should be, is there anything inherently wasteful in IEEE 802.11 that might suggest a better alternative for the job?