I used one many years ago in my tank hard to say if it really helped but with a reasonable light source and not too big a tank it certainly wont do any harm. The Maidenhead aquatics shop i visit had one in a small tank and it looked stunning plants looked the db's.
Think these are pretty crap, as the CO2 just 'sits' under the bell waiting to diffuse, this means its got a very small surface area and diffusion is therefore slow. You'd be better off getting on of the other kits.