Not sure were you intend to use it, in-game or competitive. Either way, I'm not a big fan, and you're probably not at a huge advantage or disadvantage using HA Charmander/Charizard one way or the other.
Charmander's HA is Solar Power.
>During harsh sunlight, Solar Power raises the ability-bearer's Special Attack by 50%, but it also loses 1⁄8 of its maximum HP after each turn.
This makes Charmander (Charizard, by extension) a fearsome Sun sweeper, but in exchange for longevity. The ability is further akin to having your Pokemon hold a Life Orb, higher attack in exchange for health. It is almost definitely better than Blaze, but not by a huge margin, because it has average defenses. Further, it is weak to Water and Electric types, and 4x weak to Rock, all of which are extremely common types in terms of attack, and Stealth Rock is a big threat too.
- Higher power, can nab some extra KOs.
- Works only in harsh sunlight, so useless if you don't have Sun set up.
- Reduces HP every turn
- Boosts only Special Attack, so physical moves are unaffected.
Honestly, I feel there are more cons. Further, most people prefer Mega Charizard, X or Y, which are both very, very good and preferable over regular Charizard. And on mega-evolving, the ability changes anyway. Further, Mega Char Y is a much better Sun sweeper, with massive Special Attack, and sets up Sun too through its ability.
So, all in all, personally, it's good, but not very. A trade up from the regular under certain circumstances, but definitely not if mega-evolution is taken under consideration.