Probably the biggest reason that I initially adopted MythTV back in 2004 was because of it's excellent HDTV support. I personally think that anyone spending money on new MythTV hardware should be spending it on HDTV hardware rather than legacy analog to digital Hauppauge hardware that's been around for more than a decade. I suppose reviewers keep using these analog cards because you can always add HDTV hardware later, but why start off in the hole?
The MythTV Wiki has quite a bit of information about HDTV. It's just as easy as plugging in your coax cable or pointing your UHF antenna. I find that 90% of my TV watching comes from HDTV and/or digital sources carried over a HDTV transport stream.
I use both the HD-5500 from pcHDTV and the Technisat Airstar HD-5000. These cards handle HDTV over both Air and Cable. And any recent kernel version has the HDTV drivers for these cards built right in.