We converted ours some years ago. Originally it was an extra bedroom for DS1, he loved it. But now its just us two. I have a through lounge with quite a large suite and a large tele. So we used the converted garage as a dining room. Its really good, especially at xmas. I do also keep my sewing machine on that table and do a bit of crafting, out of the way of DH, who moans at the trails of stuff i leave behind.
But actually doing the conversion was easy enough, so long as it is as sound-proofed and as insulated as you can get, with a good size radiator, you have nothing to lose.