I think it's a good investment in general. When I flip a house, I have a list of items that I can add to a home that make it a lot more marketable. One of those is HVAC, another is a garage.
One example: we did it with a tiny (2 br, < 1000 SF) house I flipped in a small town in Central Texas. It cost about $2500 to do the HVAC; that and a fresh coat of paint allowed us to turn it around for a profit of about $38K.
Some considerations were that the house was very old, very small and in an historic community, so the cute factor was pretty high, and the ability to get rid of the ugly window units was a high priority, and it goes without saying having air conditioning in that market is absolutely required. Also labor costs in that area are quite low.