I think that the offensive variants of Heatran with Air Balloon still are good, the low speed is not a problem, as Heatran has other advantages. It can check Dragons that lack a Fighting- or Water-type move and can defeat all them, except Kyurem(-B), with Hidden Power Ice. It has a Fire Blast powerful enough to 2HKO anything that doesn't resist it and has low Special Defense. With Air Balloon it also laughs at defensive Gliscor and can defeat him with HP Ice, and even if it pops Air Balloon, Heatran outspeeds and OHKOes. Despite what is said on the Smogon in-site analysis, a Timid nature is extremely helpful to outspeed defensive Gliscor and Adamant +0 Dragonite, and speed tie with other Heatran that pack HP Ice (it may not outspeed those that don't, but as long as it mantains its Air Balloon intact, Heatran shouldn't have a problem defeating them with Earth Power if they lack Air Balloon).
It is a bit harder to use, however, when you consider that is risky to switch-in on any non-Fire-, non-Poison-, and non-Ground-type attack and status moves since its Air Ballon will pop instantly. And if you want to escape from Dugtrio, it's much better to use Shed Shell. But on my opinion Air Balloon Heatran still has its merits, and I've used him properly and it is good.