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I can only speak to Baja, but I am pretty sure this applies to mainland Mexico as well. ATMs are available in any moderate sized towns and so far I have never had a problem with my card not working. I have also never had any troubles with crime around ATMs. In the bigger cities the banks often have armed guards.
If you are relying on ATMs for cash I would have two ATM cards for different banks or different accounts. That way if you lose or damage one you are still in business.
Speaking of using ATMs internationally I walked up to a random ATM in Calcutta and used it to get $300 out of my Canadian bank account. I thought that was pretty slick.
I don't pay attention to the user fees - I am sure they are not cheap, but the ATMs are so convenient and I usually take out at least a week's worth of cash at a time so overall it isn't a big deal
I was in Honduras and Guatemala for two months last year. I never wanted to to carry more than a day's cash around so I went to the ATM everyday. Smart, right? What the little receipt didn't tell me was that Bank of America was charging me $5-7 dollars each time, for being out of the country and at a non-BOA atm. However, the piece of mind I got from carrying little cash was worth it.
If you're really concerned about worst-case-scenario kidnapping schemes...don't go south of Nogales. Also, there are tons of western unions, etc., all over the region if you need to get emergency cash. Banks are universally well secured and heavily armed, so worry not. Travelers checks are also a really good option - %100 insured and difficult to cash by theives.
Use credit cards cautiously - never let it out of your sight and use only at the fancy places that look like they'll still be there tomorrow.
I would be careful extrapolating from Candian experience. It's probably the same, but there are thousands of US banks, most with virtully no profile. Oviously the larger ones are huge. The Canadian banks were 10 years ahead of the US ones in rolling out ATM services. I can remember trying to find interac linked banks in Oregon!. Just remember how long it took US businesses to get organized around debit purchases, even huge retailers like LL Bean were 5 years behind a conveninece store in Scarborough.