Over the years, they have cleaned our furnace and ducts, fixed our furnace (when we were on holiday, overseas, at Christmas time!), replaced our hot water tank (the next working day, when it crapped out when one of our three kids was taking a shower at the weekend!) and, most recently, replaced our (original) furnace, which we had planned in, because they'd been keeping an eye on it for us and advised us to move it towards the top of our to-do list :)
From my first interaction with them, it's been nothing but hassle-free, efficient and quality service; and for what I consider to be a very reasonable price. They are responsive and pleasant to deal with at all times (office staff and technicians); they turn up when they say, they provide information and answer questions according to the level of interest / savvy the homeowner exhibits (I fall into the "I-don't-need-to-know-just-take-care-of-it" category!) and they are always respectful of my home, which matters to me. Nothing worse than scratches on your floor, or having to do a whole load of clean up.
I thoroughly recommend A1. Wouldn't go anywhere else....and love that I can go to them for plumbing stuff now too!