I'm going to be getting the spell soon and I was just curious as to how permanent dead thralls actually are?

they are supposed to but sometimes it just forgets them. So other than glitches yes

Usually, they fast travel without a hitch. If you do fast travel and find they haven't appeared next to you, check your active effects. If "Reanimate Thrall" is still listed, typically you can just wait an hour (using the wait function) and they'll appear by your side. Again, 90% of the time, it works every time.

Okay, thanks! Is there anything else with dead thrall that I should know about?

If you want them to have better gear then kill em, put the stuff on them and then revive em again, they tend to actually use better weapons but armor i am unsure about

Most thralls will only wear the armor they had when alive. Not just the same type of armor, but the exact same instance. You can take it off, enchant and smith it, and put it back on them before you revive them though.

Beware, though, enchanting their armor will interfere with and negate the Dark Souls perk, so decide whether that enchantment is worth a 100 point reduction in health.

Sometimes they do, sometimes they don't...

I never use the spell because they've got a habit of getting lost.
It's annoying as hell when everyone you come across tells you to stay away
because your spell looks dangerous even though you haven't even seen
your thrall for days in-game.

This. ^

I think if you Dead Thrall too many different NPCs there's a situation wherein their corpses can be come permanent and cause lag in a cell. Hopefully someone with more experience can clarify, but I've seen people mention something about it before.