Well my Halcyon (Pro6, 18W HID) has been extremely good and worked very well. Light quality is fantastic, the beam is very tight and the whole unit is just cool. I've also used Salvo and found them to be very good in quality on all counts.
Downside of Halcyon I see is the potential risk to bulbs but I treat mine with kid gloves (though it did survive a nosedive from my shoulder D ring onto the car park at Stoney
). Downsides to Salvo I've seen is that they have had the odd flickers and occasional refusals to fire.
On both counts though, customer support is very good (network of H dealers and the always great Mr. Knackers).