The downside of installing the new software is the need to share at least some of your network bandwidth with other people. The upside is that by providing this "service" you will be rewarded by your ISP (who will provide the software) with either money (in reduced monthly payments) or other bonuses (such as free access to other public hotspots provided by the same ISP). 

The success of Cuculus' concept is dependent on two related factors: the actual security of the system and its rate of adoption. If the new system will indeed prove secure and ISPs will adopt it we might soon see an exponential growth in the number of hotspots available in each city from several hundreds to many thousands in a short period of time. One of the main advantages of such a revolution might be the quicker adoption of dual phones - cellular phones which use both GSM and WIFI. Dual phones can switch seamlessly between the two modes, using WIFI whenever a hotspot is available for reduced cost and GSM in all other cases.   

Cuculus' concept is also very appealing to ISPs who might not need to invest heavily in building and maintaining hotspots. The recently approved 802.11n wireless protocol might also help boost Cuculus' project, supporting an improved broadcast range (up to 50 meters / 164 feet indoors and 126 meters / 413 feet outdoors), having more chance to cover public spaces.