Like any big investment it is always a bit of a gamble as to whether it will be worth it or not. But... looking at the windows this morning I would have to say it is probably the best investment we have ever made. As you can see in the photos our big doors are completely dry, whereas the small windows beside the big doors (which are not double glazed) are completely covered in condensation and dripping water all over the window sills while pumping out cold air.
We were able to use our original frames and they just put the new glass into them, saving us some money, but we did choose to go with the more expensive ones rather than the medium quality ones. My advice to anyone thinking about it... they are expensive, but yes -they are worth it! And no, I'm not getting sponsored to say that!