I bought my Kobo Mini at Walmart and they have a 14 day no questions asked return policy which is pretty standard in Canada although many store will take returns after that.
As to Kobo I have no idea about longer term service but buying with a credit card gives you a two year warrenty. Walmart wanted $40 for an extended 3 year warrenty which seems excessive for a $79 device.
Amazons customer service for the kindle can't be beat IMO, but I would think Kobo would do pretty well. They are getting a lot of good press with the Glo and Mini, so why ruin it with crappy customer service.
I do love the Kobo Mini and most people seem to be very happy with the Glo. They aren't perfect, but seem (my opinion only) slightly better than most.