Canada is a good backup for PR for H1B holders in the US only for people born in India, China, Phillipines and Mexico - due to the immigration backlog for people born in these 4 countries - it usually takes 5 to 7 years to get a green card if you are born in these 4 countries to get US GC. But if you are not born in the above 4 countries - it should not take you more than 3 years to get your US GC since there is no backlog for all other countries. I am from Argentina and before moving to Canada earlier this year, we got our US green Card in 3.5 years in 2004. Most of my friends from South America (Colombia, Brazil, Peru, Argentina, Uruguay etc) have got their US green cards in 3-3.5 years - start to finish.
But from what I hear, Canada is also introducing biometrics from next year similar to what the US currently uses and so it will be easier to track who is coming in to Canada and who is leaving the country and when. It was in last year's (2008) Canadian Federal Budget and is slated for full implementation by 2011. Then it will be important to maintain the 2 in 5 year residency to maintain one's PR.