Heatran - is it any good?

Can only speculate that it will not overthrow Moltres but it hasn't been released yet so no one truly knows. Niantic can change movesets if wanted. It should be close to Entei.

Assuming the game doesn't change between now and when Heatran is introduced: t's probably going to be a great Pokémon... for people who got into the game after Moltres day. It probably won't be all that worthwhile for long-time players who will have multiple Moltres or Entei powered up, potentially a few Blast Burn Charizard and Typhlosions, etc. Heatran will be a luxury - but probably not a place to even dump Rare Candies for us.

But for anyone without numerous powerful Fire attackers? Heatran will likely be an extremely useful Pokémon.

Moltres will be the best fire type if nothing changes, because DPS is more important than TDO. That said, Heatran will be a good anchor for older players who find themselves wanting one, since its TDO will be extreme in most of fire's good matchups. That TDO is also helpful for newer players who need help with something like Tangela solo, where the available non-legendary counters can struggle with survivability.

Unless they change its moves its a bulkier Entei. Had it gotten Overheat it would be hands down the supreme fire type until potentially Reshiram or fire type megas.

I'm more so disappointed it didnt get a good steel moveset, it could have been a fantastic fairy counter with that double resistance, but instead we get....Bugbite? Really niantic?

Something like Heatran I would fully expect might have a raid day, with a much improved charge move. It's almost like they are leaving it open for just such an occasion.

This is the normal way things are. The biggest baddest of the type doesn't get the best moves in general. Rayquaza with Outrage, Moltres with Overheat, Moltres with Sky Attack, Raikou with Wild Charge, those are exceptions.

So Heatran getting fire blast (like Ho-Oh) doesn't surprise me. It should be quite effective, even without the stronger damage that Overheat does.

If the strongest pokemon of a type got the strongest move, there would be zero reason to use anything else. It would make the game very one-dimensional.

Simple answer: Heatran got fucked. (Sorry to be the giy to say this)

Long answer: It will be a below average bulky fire attacker, maybe it could fit in the sixth slot of a Fire lineup instead of Entei if it's a bit too frail.
Unfortunately, Moltres, Flareon, Blaziken and Charizard already exist, and not to mention that Moltres has Sky Attack.... Not to mention that it lost its role as a generalist, kind of being a jack of all trades kind of pick. It could roast DG Blissey's alive and all those damned Fairy types. However, thanks to Fire Blast, it can do none of that. It requires Rare Candy, and its borderline useless. Not Regi status, probably as meta relevant as Latias, dare I say Latios...

Offering counter points to answers here, Heatran will not be the best Fire type even if it got Overheat. This is because 65% or above of relevant Fire type matchups are against Grass and Bug. And Moltres has Sky Attack, which is better than Overheat. It also double resists both of those types, bringing its bulk to about on par with Entei.