At ATM's, yes without a problem, but check what your bank charges as a surcharge (BMO has their own inflated exchange + other ATM fee/foreign ATM fee). It might be worth buying currency before you go. I wouldn't just go and grab a $20 from the ATM for what they charge you, it's not worth it.
In some stores you may find into problems using a debit card because a lot of banking instituations in the states have visa's and mastercard's tied to personal bank accounts, therefore only credit cards are recognized.
Hope this helps.