Pros: I use this for mtb'ing through the woods in the evening now that its autumn here in the UK its dark. This thing is like the xenon's on my audi (ok well maybe not quite...but close). I can do just as quick as I can in the day, through twisty woodland, with one of these on the bars an a P7 torch on my helmet.
I rode with it in the pouring rain the other day, no problems with waterproofing.

Cons: its like a car headlamp on high beam. If you have it on full in the rain - it will reflect off the rain and dazzle you. But thats really a pro, because its so bright.
The O-ring mount so far has shown no sign of slipping,even taking some gnarly tracks with at least one wet-tree root wipeout (sideways rather than over the front). But I expect the o-ring will break at some point, then I will have to find another from somewhere. Nice if there was a spare in the pack.
The mount is really handlebar only.

Other Thoughts: I put it on the lowest setting for riding on the road towards the woods, it seems to not be too annoying for drivers.
The battery pack has a velcro wrap which is big enough even for a fat mtb top tube. It has buckles as well, but no strap is provided. I put a cable tie through the buckles around the top tube as well, as an insurance against the velcro coming away.
There have been some comments about build quality on other boards, but it seems fine to me (as someone who doesn't take these things to bits, I just used them).

Bottomline: I looked at all the brand name competition, nite rider etc, in this price bracket, nothing comes close on brightness and duration. Combine it with a P7 torch (sku 12623) on your head (this particular light doesn't have a head mounting solution but is great on the bars) and you have a perfect low cost MTB night riding rig.

Pros: Very bright
Good build quality
"Medium" setting useful on the road
Light weight allows use as helmet light
Long lasting battery on medium
Flashing light perfect for daytime visibility in heavy rail
Easy to change lens color
Inexpensive

Cons: More mounting options needed
Need to include helmet mounts
Should include a "spot" reflector
Need switch instead of unplugging battery cable
Ordering/shipping time is long

Other Thoughts: Half of my commute is in the dark and I should have got this light a year ago. The small size of the battery allowed me to jury rig a helmet mount (should be included). Helmet mounted, I avoid blinding traffic and any pedestrians just by a slight nod of my head. I use the medium setting that gives me 5+ hours of battery life. I use the high setting on rainy mornings on dark roads. The flashing light is useful for cars seeing you during downpours during the day. The light seems fairly waterproof but the lens did fog some in the rain (easily fixed by unscrewing the lens and cleaning it. It would be nice to include a green lens for better visibility and a spot reflector for a more focused beam. Should have a switch instead of unplugging the battery which will eventually wear out the plug.

Bottomline: Get it now! See and be seen.

Pros: For the price this is an amazing light. I was actually very pleasantly surprised by the build quality. I bought 2 and pulled both apart to check out the insides to make sure everything was ok. If anything, there's probably a bit too much metal making the light just a touch on the heavy side. However the heat transfer seems good. Everything was put together nicely, well soldered and the fit of the various components was very good. The brightness is insane - locally I'd have to pay at least 3 time as much to get something even close. This thing really lights up the track.

Cons: The flashing mode is useless. It feels like I'm in a disco. It would have been nice, others have already commented, to have a lower level mode when riding around town.

Other Thoughts: This light has quite a "floody" beam. It would be nice to have a model available (or perhaps just a reflector option) for more of a pencil beam. I'd like two lights on my bike - a flood and a spot. Plus two lights are always better in case one dies and your still out in the bush.

Bottomline: For the price, you can't go wrong. I've already ordered 3 more for friends.

Pros: Gets people's attention
Inexpensive
Lithium battery technology

Cons: Status LED stays lit when light is off (bad for attracting thieves)
Beam too focused -- irritates pedestrians
Elastic band mount not sturdy enough

Other Thoughts: This is the first bicycle light I have used that elicits complaints from people on the street. I light to grab the attention of cab drivers but the hostility of pedestrians is a bit too much.

Bottomline: Great light for the money. The strobe mode is bit too intense for city driving.