It depends a lot on screen size, but SD content looks generally very poor and fuzzy, even compared to HD. Put something with UHD on and then watch SD and you will want to scratch your eyeballs out.
It was enough for me to fork out the 60€ a year to get the HD+ card, so that I can watch the commercial channels in HD. On the old 32" FullHD TV, you would notice a definite difference, but the difference wasn't worth paying money for. With UHD it is having to do pretty much 5 times the horizontal scaling and 4 times vertical scaling to get from an old SD image to UHD. Even with the best scaling, it is going to feel blurry. I haven't tried a DVD yet and we have all HD channels on the TV now.