It isn't bad, in my experience, though not as temperature-resistant as some newer powders.
Note that I use the term "resistant," not stable. ALL powders will gain some velocity at temperatures over 70 degrees, but the more temp-resistant ones will gain less.
In cold weather, many newer powders will produce just about the same velocities at least down to zero degrees Fahrenheit, and often even lower. Some other powders will lose considerable velocity.
Also, even temp-resistant powders will sometimes lose or gain more velocity than expected when used outside the pressure they're designed for, or in cartridges where another powder would work better. One of my friends at a major pressure-lab used the example of Hodgdon Varget with 140-grain bullets in the .260 Remington. There are much better powders for that use, and even though Varget was the original Hodgdon Extreme, it doesn't act like it when misapplied.
