So Unstoppable is situationally useful, depending on your tank build, and some of that depends on your class and what content you run. If your class has no other source of Major Resolve/Ward, then it might actually be worth it all the time. Otherwise, I rarely need immovability except for certain fights (its nice on Molag Kena, but I just use pots), however, I don't tank trials so I'm not sure whether or not its useful there.

I'd suggest you look at what class and content you want to run, then decide if Unstoppable can be replaced with a class skill or use an Immovable pot when you want it, or if its going to fit with your build.

General Tank Advice

Class: So you don't want to use dragonknight. That's cool, all classes can tank. Probably the next best class is Warden, but really all you need to tank is a taunt, crowd control, and survivability, so any class can be a tank with practice.

A good tank's job is to stay alive, taunt the boss and priority adds, crowd control the rest, debuff the boss, and buff the group.

For staying alive, you want enough health, physical and spell resistance, and sustain to stay alive. How much of each you need depends on the content you are running. Unless you are building for pure health, most pure tanks run around 30-35K health. The max physical and spell resistance is around 32-33K for 50% damage reduction, but hitting max here may or may not be necessary for the content. You do need enough sustain to block as much as you need, which again, the amount of blocking/perma-blocking varies. A trials tank needs to block a lot more than a group dungeon tank.

In combat, you'll focus on taunting the boss and priority adds. There is no AOE taunt, so you can't hold aggro on everything. At best, you can grab the mobs' initial aggro with an AOE at the very start of combat, but they'll reaggro in approx. 7 seconds if you don't taunt them. So your priority is to taunt priority adds (usually 2handers or miniboss adds) that can one-shot your DDs or healer. As for the other adds in the mob, you can crowd control them with lockdowns or slows for your DDs to mow down.

Obviously in a boss fight, you want to taunt the boss. I prefer to use Pierce Armor from One-Hand and Shield, which also has a powerful debuff. A good tank will also keep the boss as still as possible because most DPS comes from ground based AOEs. Be alert to block, bash, and interrupt. Otherwise, boss fights are more about dealing with the boss mechanics and keeping the boss taunted, debuffed, and standing in AOEs for the tank.