Going half illithid is mostly about getting cool powers while keeping the story intact, which is pretty nice if youre not worried about the whole moral thing. Dark veins start showing up and your eyes get weird but honestly most folks dont care except Lae'zel who definitely lets you know shes not happy. The real good stuff comes from those sweet illithid abilities like flying and free casting, plus youre better at talking your way through things which is always useful. Interesting that you can use those fancy [Illithid] options in conversations to mess with peoples heads a bit

The whole transformation thing doesnt really lock you out of anything major until right at the end, when going full illithid becomes a different story entirely. Most companions stick around even if theyre not thrilled about it, just dont go full ceremorphosis unless youre ready to lose everyone and basically destroy yourself. Worth noting that the powers are seriously strong though, so if youre after a power boost without much drama this is probably your best bet