Is Entei still a good investment?

Some people questioned the value of it already in the wake of Blast Burn Charizard. I wasn't one of them, but without an immediate target for it it's hard to justify doing so. Keep in mind we'll likely be getting a Moltres day following the Yokosuka event, who is already the preferred fire type legendary who won't be burdened by the L15 research cap.

Heatran on the other hand - moves pending - will blow both Moltres and Entei out of the equation. I like fire doggo but I can't recommend sinking resources into it.

Fire never was a good investment. It recently had a brief use against registeel. It's 15 minutes of fame past and I don't see any use for fire pokemon coming soon.
If you already have BB Charizard, moltres or another entei (or even a bunch of flareon) you shouldn't. Save your stardust for the time being.

Spend stardust and possibly rare candy on what you have a need for right now. If you don't *need* Entei, and you don't need Entei *right now*, don't put resources into Entei right now. Unless your bag is overstuffed with rare candy and you need to convert it. In which case you'll have plenty for Heatran (or whoever) if and when the time comes.

Entei is by no means a bad Pokémon but not really a necessary investment. It’s has few edges over Moltres. First, ice types, but it’s already covered by Machamp and SD Tyranitar. Second, two bar flamethrower, Charizard has Blast Burn and Flareon has flamethrower as well. Last, bulk. Although it’s unmatched in tdo, dps-wise it’s flareon with less deaths.