I took the plunge recently, and joined the HD world.
I bought a Samsung UE40D6100 40 inch TV. It's lovely. The 3 or 4 Freeview channels that are broadcast in HD look very good indeed.
However, what is perhaps not advertised so much is that immediately you notice how bad all the other standard definition content is.
Suddenly, you notice (or notice more, anyway) blockiness around edges, and various other things.
I'm one of those people who are quite sensitive to image blockiness, or low sampling rates in audio, clipping, that sort of thing.
Some content starts to look very very fake indeed. This is known as the Soap Opera effect.
I've also noticed the audio getting out of sync with the video, which is something I just can't stand.
Most of the problems can be alleviated by disabling specific settings on the TV.
So, on the whole, a good thing. But there are downsides.