I'm another one who has used a number of the Tesco lights in the past, and had run two connected in series. I'd sawn the barrels off and soldered in leads, to then either connect to an external battery pack or a dynamo. When running them off a dynamo, unfortunately they're not as bright as the original battery configuration.
The disadvantage of using a battery pack is of course the extra weight and the fact you've got to find some way of securing them in a waterproof way to the frame. By running 2 lights together, in order to get a decent run time I ended up having 3 sets of 5 x rechargeable AA batteries in parallel, which gave me about 4-5 hours of light. However, it did give a good spread of light.
Since getting my new bike, mainly for aesthetic reasons, but also as it would be slightly easier to change batteries, having looked at a number of alternatives, the best compromise on price, power output and flexibility was the Hope Vision 1, so I got 2 of these which give a similar spread of light to the Tesco lights, without having to resort to using full power.
The main advantage of the Hope lights compared to other high powered bike LED lights* are that they take AA batteries, and so if they run flat, you're not left carrying a heavy useless battery pack, as you can easily fit some new batteries.
You can get Hope's for less than £70, and they're an excellent piece of kit.
* Cateyes do not fall in this category as their output is much less.