I've been running a Quark AA2 with a Towfish Lockblock for about a year. In dusk/dawn conditions I'll usually put it in strobe mode and have had many unsolicited comments from runners, cyclists and drivers that it's extremely visible, even from 1/2 mile or more away. When it's dark it goes on full bright mode, which is fine if the ride is up to ~2 hours in the dark.
The only downside is that the batteries rattle a little bit. Not loudly, but I like a silent bike.