I'unno, I like 'em. But like most glamorous units, they're severely expensive...
With W9, they CANNOT BE HIT... You're looking at 23+ defense and blessed quickness on top of a free mirror image that gets a slight boost thanks to their skill in air. And if you've got E8, they're fatigue neutral except for the double-rounds.
The hersir have 2 attacks at 14 value and 12 strength (hersir also have the light lance, so they get to put those 39 movepoints {when bless-quickened} to use), so they're generally pretty competent at just smacking stuff around the old-fashioned way. The vanjarls, however, can just run around spamming leeching touch. If you're into that sort of thing, of course...
The most common thing that gets around their obscene defense values are projectiles, but, hey... All of the commander vanir can cast air shield.
And that's just the stuff they do bare-bones naked. Oh, did I mention that they all have glamour and thus are seriously stealthy, so they can form black ops squads behind enemy lines? Yeah. Also sailing and 16-17 MR.
They're fun enough when just thugging them out normally and sending them out along with the rest of the sacred stealthy cav, but considering you get one of them as the first commander in your PD... They're a really sweet deal.
I'm sure there are other, better ways of using them that I'm not privy to... But I certainly got good usage out of a water/earth bless.
All that aside, EA Helheim has brilliant corpse construct potential. You've got a dedicated A1D1 sacred caster at your home base, another mage with E2-3 for hammers and lightning rods, and plenty of air access for searching or even building storm staves, if you're into that.