First off my Blackburn Mars 4.0, which I've been using a hell of a lot on road and off road for the past year. In the past couple of weeks, the on/off switch started to act up (not switching on, not switching off etc.) After checking out their website I discovered they offered a lifetime warranty for all products. The Mar's rear light is the best one I've ever had so I decided to chuck the broken light in an envelope and post it to their offices in Illinois, USA. To be honest I was not expecting anything back, but a week later, I get a box delivered first class with a brand new Mars light! OK, the Blackburn light is not cheap, but in terms of performance and customer support, its well worth the money.
Next problem I had was with my Hope Vision 4 battery. I think it got a knock or I dropped it, but the battery inside the battery casing was loose and a couple of times the light did not switch on. I was inside the warranty period, so I emailed Hope support explaining the problem. Within 5 minutes I had an email reply saying to send in and they would check it out. So again, battery in envelope and off to the UK. Again, within the week I got a package in the post which contained a new battery, and the newer version too (which is that bit slimmer).
Anyway, moral of this post is if you need a rear light go get a Blackburn and a Hope if you need a front light!