I've been using Stan's Grail rims for a year, with Hutchinson Sector 28 tyres. Minimal loss of air - well, my idea of minimal anyway, since I pump them up before each ride and it varies from 1-2 psi 'needed' after a day, to 5 psi after 4-5 days. That's with pressures at 70 & 80 psi, front/rear. All four tyres I've fitted on the Grails have gone on without levers (and I'm far from expert, having only been cycling 18 months) and all four have inflated first time with a track pump.
So, I have no comparators, but the Grails, with those tyres, seem to work very well in all respects.