ToME: the Tales of Maj'Eyal

I've played my fair share of TOME—and have recently had some wins as a TW. I'm interested in playing a mindslayer (had a few iffy runs), but don't quite get how mindstars work and how to use my telekinetic grip effectively...is that just to wield cool weapons I normally couldn't utilize? What should I be aiming for in the main hand? the off hand? Any help would be much appreciated.

Mindstars offer mostly caster-y stats and are only viable for melee use after buying psiblades in zigur (it's a tree with a sustain that works like their weapon mastery)
You would normally put a 2h weapon in your psi slot at all times unless you need to meet requirements at which point you can put a gem there, and a 2h weapon in your hands, you don't really use mindstars at all for mindslayer.
If you REALLY want to use them, they still definitely don't go in your psi slot because that's awful currently, you'd use one in each hand and only once you've gotten psiblades purchased from zigur, and no sooner.
Keep in mind tk weapon is affected by mastery, so even if you go psiblades you want to put points in weapon mastery, that's one of the big reasons why I prefer to just use regular weapons to make the most out of it and not use 10 generics on weapon masteries.

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"As dying is one of the leading causes of death, you should avoid dying." -rekenner

"I'll bond with a cactus until my buttcheeks touch the sand before I play nethack again" -Gagarin

Also, Psiblades is a Wild Gift. So don't bother if you are Undead.

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If not mindslayers, what class utilizes mindstars effectively?

Oozemancer uses them. They are quite powerful weapons for classes that max Willpower and Cunning. Unlike normal weapons, they don't have an offhand damage penalty. They also have a large amount of armor penetration. They generally do more damage than a 2H weapon on Mindslayer and give defensive talents such as Leaves Tide. The main issue is having to spend extra points on mastery, as Sheila said. The other issue is that mindstar egos are generally bad compare to regular weapons.

Also Slolipsist and Doomed.

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You can also have great Wyrmics with mindstars.

Some actually like to use a 1-handed weapon (2-handed if you're an Ogre) and a minstar on the offhand, because there's no offhand penalty. This option gets the most use on melee classes that have mindpower to use them, like Cursed or Wyrmic. Psyshot and Possessor use this setup as part of their builds (the latter as an option for attacking talents).

I actually like using Mindstars and a TK weapon in MS, but that's the thing about ToME. You can customize your character any way you see fit.