I have the Dive Rite 700 watt canister. I think the rated burn time is 4 hours, but I am getting easily 6+. It's not a 21w HID, but I'm happy with it.
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Seems like everyone is switching over to LED, I've just heard that Halcyon is D/Cing their HID line and going all LED.
Henrik...I suspect we're going to have to wait awhie for some official news from Halcyon...
I would recommend going with a slightly used 21W over a new 10W because you'll save yourself the upgrade fever down the road (I find the 35 and 50W HIDs to actually be too bright)