A conservatory done well can add instant value to your home. However, beware the, "greenhouse effect" in the summer on south and west facing conservatories. The heat and stuffiness inside could become unbearable and you will not want to enjoy your new space, using it to perhaps only grow your tomatoes instead! A north or eastern facing conservatory is perhaps a better location.
Heat loss in the winter could also be an issue - so using low-emissivity glass is perhaps the best option to ensure the most even temperature control, reflecting the heat during the hot summer and keeping the heat in during the freezing winter months.
Done well, your conservatory could look amazing especially with the latest high-tech engineering and glass technology.
There are many types of conservatories, for example - traditional hardwood, glass cubes and high-ceilinged conservatory extensions and you need to decide which would be the best option for the style of your house.
It is best to check first if you need to apply for planning permission or building regulations and also conservation and listed building consent.