I'm using a Freecom one without any problems that I got for £30 ages ago.
The supplied mini ariel is utterly useless (where I live) and couldn't pick up a single thing. Luckily you can connect a roof top ariel and once I did I was able to get every single channel.
Picture quality is ok for me. In a window the quality is excellent but go full screen and it does suffer some loss. This is purely down to the fact that digital broadcasts are quite low in resolution (around 5xx x 4xx or something similar) and my display has a native resolution of 1650x1050. The image isn't upscaled but stretched, hence the loss in quality.
Make sure whatever one you get it has BDA drivers! If not, then you're stuck with using the supplied software. BDA drivers allow the device to work with 3rd party programs like Windows Media Centre and other open source alternatives.
A PCI card is the better option but I have a small Shuttle case with the PCI slot occupied by a sound card. I only wanted something just so I could watch a bit of TV now and again as theres no space for a TV in my room. For that purpose, this USB tuner has been fine.
The added bonus is I can I can use the tuner on my laptop and PC.