Is Tentacruel worth it?

There's better Pokemon for attacking into Fire types, like Vaporeon or any Water Gun user pretty much. Also their quick moves suck on offence. 1s moves aren't fast enough to squeeze in an extra attack before needing to dodge, so 3 x 0.5s moves would do more damage or one of the longer ~1.4s moves since they'll generally have 15 base damage.

For defence they're alright, the poison typing resists grass which makes Tentacruel neutral to it, so there's a few less potential attackers compared to pure water types. However there's usually better Pokemon that would use your stardust investment more effectively.

I have always liked using tentacruels with non water attacks to train against vaporeons in gyms. Just have to dodge the charge moves.

Imo acid is better cuz you can attack x2 between dodges. I use mine with acid/blizzard to prestige on Dragonite. Im currently trying to replace my training Tentacruel team with a Cloyster/dewgong team. Its a tanky mon but I prefer using Hp starmie and vapes over this mon because WG is OP.

Blizzard is the move you want for defense. can't really think of a reason to use him for attacking.Sorry for your grinding. I hope you hatch some more and have better luck, he is a cool Pokemon with interesting moves, looks, and type. One of my favorite "fun" Pokemon to put in gyms.

In theory I like him a lot and I personally prefer Sludge Wave on him because that puts him into an interesting position of having a ton of useful resists while having a largely unresisted STAB moveset. That makes him kind of the swiss army knife of attacking, just don't send him after Rhydon. Also much like Snorlax, he is super tanky, but don't expect him to kill anything quickly. If you want fast wins, look at a different pokemon.

In Practice, Poison Jab feels way too clunky for me to rank up either my PJ + HP Tentacruel or PJ + SW Tentacruel.

My recommendation is to try him out and see if you like him before spending the dust.

While I actually like Tentacruel, and think he is definitely underrated, I:

A) prefer other movesets. Namely either Blizzard, or full poison set

B) would use him with attacking in mind rather than defending (CP already too low, will only get worse with gen 2)

and

C) prefer not to spend dust on him. Since you'll be eyeing him up as an attacker, and he's not a top tier one but good depth material, I'd rather keep evolving decent lvl 28-30 Tentacools I find until I get the moves I want - saving a ton of dust while being 95% as effective

For example (granted I've caught a ton of Tentacool) I have all these at level 30 without ever spending a single dust:
1968 CP 71% Jab/Sludge
1965 CP 80% Acid/Sludge
1911 CP 71% Jab/Blizzard
1898 CP 67% Jab/Blizzard
etc..

For offense I like Poison Jab + Hydro pump, but there's normally a better attacker available (good for prestiging though).

Defensively I prefer Poison/Blizzard.

He's a good Poke but one I tend to use as a change rather than as a first choice.