On the plus side- HVAC has always provided a living. I have never had to collect an unemployment check. I was laid off once in twenty-five plus years in the trade and had a new job within a week.
On the minus side- If you are older, you might want to consider a different field. There are physically-demanding aspects to the trade as well as wide variations in outdoor working conditions.
The HVAC business itself has changed a lot since I first started. Unfortunately, it has become less focused upon quality and more upon increasing profit margins through "short-cuts", hidden charges to customers, and an overall emphasis upon sales as opposed to service.
Also, there is a trend toward hiring lower-wage "parts changers" as opposed to truly skilled tradespersons. If there is a call-back, many companies will charge the customer asserting that it was a different problem to the equipment previously serviced. So, the unskilled or semi-skilled tech can find ready employment. No doubt, you've seen the newspaper and Internet ads proclaiming a 30-day course of instruction that allows you to become a "certified" HVAC technician. I've also seen ads from HVAC companies such as "Seeking a Career Change? We Can Help!" Basically, they state that they, too, can have an individual right off the street and into a service van after a few short weeks of training.
I also agree with an earlier post that there really is not much of an opportunity for advancement in this trade. Sure, over the years, I've had individuals in companies "blow smoke up my skirt" by telling me that they were considering me for for future service manager positions that never materialized; in fact, there were never going to be any future positions. It was just a line that various companies used in an attempt to retain their senior technicians (other senior techs were told the same things). I finally told one supervisor, "Look! You don't have to keep telling me this bogus crap. I know and you know that there are not going to be any service manager positions in the next ten to twenty years. I realize that I am much more valuable to you folks out in the field and you have no intention of promoting me. I'm satisfied where I am at, you don't have to lie to me!"
The people that do seem to advance in the trade seem to be the ones who either have relatives or friends in high places or know how to "push all the right buttons" in curry-favoring a position. So many SM's have a pitiful lack of service knowledge and an even greater lack of people and business skills.
It's unreal and you wonder how so many get away with treating both internal and external customers the way they do.
Negatives aside, there does seem a certain degree of stability in working within the trade. It's not like being a factory worker or a mid-level manager in a corporate environment where you can be down-sized and it might take months if not years to find a similar position. In HVAC, there is always a place for an experienced and talented technician.