This is my understanding of digital TV reception quality - and feel free to correct me:
Digital TV is "perfect", high quality (dvd quality) reception. As it is made up of 1's and 0's, the reception is either there or it isn't. Thus, the quality doesn't deminish like it does with standard definition/analogue, it either displays/recieves or it doesn't.
I have a Twinhan DVB-T card, and the quality is perfect all the time, but occasionally I get "low singnal" warnings and the picture/sound drops out.
To test whethere you are likely to have problems, if you live in an area with constant good standard/analogue TV you should recieve constant good highdef/digital TV.
The quality of cards is another issue all together, but as for reception it is a very black and white issue.
As for cards, I'd avoid Twinhan - their cards are alright, but I am not satisfied with their software overly, but they are an alright cheap card. I'd personally look at a Fusion/Ultraview card by Divco or the DNTV Live! cards.