I would say if you have most of the bits already go for it!
I have TV Tuner in my HTPC, although I have never got to the stage of taking it downstairs and using it as a HTPC, so the card is totally pointless
I think the trouble is I got a Squeezebox at about the same time, and I prefer that, but XP MCE is pretty cool for photos and videos.
If you are thinking of mythtv there are few people on here who are better placed to offer advice.......andym......
Cheers
Cris
PS - you never said what tv you have, what input are you trying to use?