I use these arrows a lot and they are BRILLIANT.
Well worth the money IMHO.
Save your mages spell slots for things other than breach, dispel magic, spell thrust etc, because with these arrows you don't need any of the spells that do these things.
On a successful hit, ONE ARROW dispels all magical protections from spells and potions (although not equipped items) that your enemy may have carefully spent time casting. Then just switch to ordinary arrows and pepper them (or slice them) while they stand there looking confused.
They are expensive because having too many of these would seriously affect the game balance. If you don't believe me, just give yourself a stack using CLUACONSOLE (item code AROW07) and see how boring it gets after a while.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas