So far, most major airports which I have visited like Abu Dhabi, Dubai, SFO, HK, KL, Shanghai, Beijing, Tokyo have free internet access. Which is good enough to check emails and surf the net.
Changi Airport is ok lar but can be improve further for those visitors who like to use their own personal laptops instead of those free kiosk which is only 15 mins of usage. I would call it a world class airport but not world class internet connection.
As for Wireless@SG, I really do not know why some have signal but no access. They say it's free usage but only certain hotpots are workable. I rather use 3.5G Broadband as you can surf almost anywhere.......without interruption with a price to pay lar.