The MJ-872 is very bright but suffers with overheating issues. It seems to not have enough cooling surface area to drive the 4x XM-L LED's without issues. In which case, after 4 or 5 mins of riding it automatically steps down to the power level below to prevent overheating.
Of course this may not happen on really cold nights if cycling fast, but ive experienced this issue on several units.
dont get me wrong though, its very bright, even on the power level below it outshines a lot more expensive lights!.
My advice would be to go for a twin XML such as: http://www.ebay.co.uk/itm/Inton-2x-XM-L ... 145wt_1139
2000 lumens (apparently) but boy is it bright. id say about 10-20% brighter than the MJ-872 on full beam.
That particular seller also reviews lights on the following link:
http://www.torchythebatteryboy.com/p/bi ... abase.html
Lots of good info on there, and he offers really good customer service.
The best budget light hands down however is this one:
UK supplied by an enthusiast, great customer service, all checked before despatch (batteries are higher quality than usual rubbish), and rated at 1000 lumen, all for £67!
It has a more 'spot' light effect and as such is perfect for helmet mounting, but if on a tight budget you really cant go wrong with that one. For more 'spread' go for the twin XML. For a cheap awesome set up, go for both!