I bought the Terra 2 from
chain reaction for £79. there a brilliant bit of kit. I was really impressed with the build quality.
Downsides are that the lights don't release from the mounting brackets so you have to remove them from the bike if you leave it anywhere. The fact that the brackets are designed to be a tooless set up is a plus and a minus! The battery is also a rather large lump which you have to Velcro to the frame. On the upside it is a li-ion battery.
I don't ride off road, so can't comment on their off road capabilites, but on the rural cumbrian lanes I ride they are superb. I can confidently ride at 25 mile an hour.
Personally I would buy the Terra 2 over the 1 becuase the two units combined provide a nice spread of light, though one unit on it's own is little too weak to ride fast.
One unit is designed as a spot and the other a flood. They've also proved really waterproof as well, surviving some torrential rain showers earlier in the winter.
How I get on with them over the course of a winter remains to be seen, but at the moment I'm really impressed.