Uptime Robot works pretty fine for me. Sometimes the UI is a bit buggy and slow, but it gets the job done. This is not perfect, and I am still looking for a better alternative
From what I can see, there is around 40% of my website visitors are using IPv6, things are looking pretty good actually. IPv4's prices are starting to go down. 3-4 months ago an IP block can sell at $50+ per IP, this is now in the $40-50 range.