Limescale in bathrooms is one of the hardest things to tackle and a bugbear for modern bathrooms since chrome, glass and high gloss surfaces are so popular in contemporary bathroom installations. Spotless have found <a href="http://www.pgpro.co.uk/viakal-professional-anti-limescale-spray">Viakal (P&G)</a> to be the most effective treatment for limescale removal and gives great results quickly. Whilst it's comparatively expensive, it tends to go further if applied correctly. Note that it should NEVER be mixed with other cleaning products as is always the case, but it is particularly dangerous when mixed with bleach. Always ensure the area is well ventilated. Regular cleaning will keep a build up at bay.
A great tip for limescale on shower screens with a really heavy build up is to use a decorator's scraper with stanley blade to remove excess scale before starting. Removing the bulk of the scale before you start scrubbing is a great way to save time. Always test a small area carefully first and be careful not to scratch the glass. Don't leave the Viakal on the metal to dry before rinsing as it will potentially damage the surface finish. This obviously wouldn't be appropriate for patterned glass.
Another important point which a lot of people don't consider is the quality of the fittings in your kitchen or bathroom. Limescale will accumulate more quickly and more severely on cheaper metal fittings. The reacitivty and quality of the metals dictates how resistant they are to scale build up. If you're considering installing new taps or fittings, bear this in mind for longer lasting aesthetics.