I have used several low-end light systems.
I have both the Smart and Electrons (which are at a good price at
Allterraincycles).
The Smarts cost me £25 (SJS on
eBay) so are about the best you can get for the money. They are heavy, the bracket flexes quite a lot (its secure, but they bounce around a bit), and the battery needs careful charging (I got a £3 plug-in timer to ensure I didn't overcharge.
The Electrons are smaller, neater and have a much lighter and better NiMH battery, and also a better mounting in the bottle cage. Downsides were the lamp mountings tended to loosen so the lights went floppy - easily fixed with threadlock, and the plugs pulled out of the lights fairly easily - fixed by swapping l light with r light so plugs were on the inside. Apart from this I had no problems with durability.
I have also used a Cateye ABS 10 (mounting fell apart and lost spring).
I'm now using a Dinotte 5w Ultra.
Overall, for around £40 the electrons are probably best. The Dinotte was £85 and worth the extra.