Short answer: US experience is useful, better than other countries, but not as valuable as Canadian experience.
Long answer: It really depends on whether the company recognizes the US experience. Canadian companies have a lot of national pride, they would rather take people in who were educated in Canada and work in Canada than those who go to the US. Why do you think the Canadian government is pushing immigration of skilled workers? It's because they're experiencing a brain drain of their own citizens to the United States. It is easy for Canadian employers to check up US references because calling the US is just a local call and they have access to certain US databases where they can verify employment and company existence. A lot of US companies also operate in Canada and there is some name familiarity too. Experience with a large international US company is way more valuable than a small local company because of name recognition. Having worked in the US (and I guess to some extent the UK, Australia, NZ as well) also shows (although sometimes not really accurately) that you have some proficiency in English and have had experience working in an English-speaking environment too, which will give you an advantage compared to experience working in a non-English speaking country.
Canadian employers are also more familiar with the Canadian education system and trust in their universities and colleges. The main importance of Canadian experience is that they want you to be familiar with Canadian customs, culture, policies, laws, and codes, etc. and while there are many similarities between Canada and the US, there are also quite significant differences. For example, for an accountant there are many tax laws in US that do not apply in Canada and vice versa. For a telecommunications engineer, there are policies, standards and frequencies that differ from that of the US especially in infrastructure and dissemination. You could be a history teacher in the US and you won't be able to teach Canadian history without having to learn it yourself. There are also differences in workplace culture. For example the US adheres to affirmative action policies, Canada does not. Canada has much stricter labor laws than the US. Canada has laws preventing discrimination based on sexual orientation, but the US mostly does not have such laws. Basically the US is not Canada, don't expect Canadian employers to treat them like the same country. The Canadian federal and provincial governments, as well as companies that have big contracts with them, tend to hire those with Canadian experience out of principle.
My personal experience: I have a Masters and Ph.D. from US universities, am well-published, speak perfect English, have an outstanding teaching record and my doctoral advisors are world-reknowned in their field. I worked as a professor in the US for 5 years at an well-known institution and then spent another 5 years as a consultant in NYC. When I applied for professorial positions at universities and colleges here in Canada, the unofficial reply I got from a couple of universities was that they give priority to Canadian Ph.D.s.. So I started looking for other jobs outside of academia. I have all the relevant technical certifications and sent out close to 500 resumes over the last year and not one reply. I worked for CGI in the US but CGI in Toronto wouldn't even consider me because they said I hadn't been in Canada long enough to pass security clearances for their government contracts. Employers wouldn't even bother to call my overseas references, but they were still willing to call my US references. Even when I applied for retail jobs at Home Depot and Walmart, they wanted me to have minimum 1 year of retail experience and they wanted Canadian references. So I went to employment agencies for temp work and they made me take many clerical and typing tests to prove my skills but they were eventually willing to give me a shot. My first job in Canada was as a temp doing data entry where I met many immigrants. Some had advanced degrees from the US and they were working with me. But working that job helped me with a reference to obtain my current job, which is still a survival job providing tech support at a call center. But I performed well at these "survival jobs" and recently my employer commented that I had performed really well and received a mini promotion. Over the last year I have built up 2 strong Canadian references by starting from scratch, taking whatever job would come by, and it has definitely padded my resume well. I recently sent out my revised resume to just 10 companies and already got 2 interview calls. I feel that I have benefited from the experience working for Canadian companies and am humbled by it too.
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