It mentions "momentarily" in the description, but that's not exactly specific. I'll mow the lawn momentarily, but it might actually be tomorrow
Have things like Great Weapon Master come up yet with Ethereal Weapon? How did you adjudicate it? It says that it still does "additional effects", +10 damage is an additional effect to me, so is it kind of free (no -5 to-hit, just the Dex save)? No +10 damage on a saved half-hit either, but there's no reason to not try for it? That seems the easiest way to play it, but it's probably too good.
Or would you give +5 to the enemy's Dex save (which is massive)? This would probably over-nerf the feat/power combo.
GWM seems *very* good with ethereal weapon if it's the former, bad with the latter, but I can't think of anything in between that's actually stated anywhere to resolve it reasonably. Well, +2 to the enemy's Dex save would do it, but that would be me just using my godly -/+2 powers to resolve any situation.....
A V.human with GWM, Immortal 1 into Fighter 2 might be a very strong starting mêlée build depending on how it's played. Then go for BM dice or more Immortal psi-points (or both) down the track. As a gestalt they'd be insane, but pretty good as a normal character too. Psi points cap your Ethereal spam, but GWM is free and is pretty much a small smite for whenever you do ethereal it. 4 small dex-save smites at lvl1 is a pretty good starting block, you could go anywhere you wanted with the build after that.
Combos with Hex(Dex), AS, BM dice, smite, SA, ass-stab, all nice damage boosters. 1 psi point for anti-AC +10 damage seems pretty good on the resource management front, with Mystic psi scaling in silly ways to make it worthwhile levelling in it to spam ethereal.
Ethereal GWM seems very viable for minmaxing stuff.