Having a HDTV, here are my thoughts.
I have a Toshiba TW40X81, a rear projection 40 inch 16:9 HDTV.
First off, HDTVs can all view "regular" tv signals, such as cable, etc.
A projection HDTV looks a hell of a lot better than a normal projection TV, because of line doubling. Regular TV is interlace, which is why you see scan lines on a regular TV. A HDTV does not have this. It recomposes the image to be non-interlaced, or progressive, so that the scan lines are invisible.
The BIG thing about having a HDTV is that you can view anamorphic DVDs in anamorphic mode. This gives you a picture that has up to 33% more resolution than a normal TV.
Most people are buying HDTVs because of this, not because of OTA programming.
Trust me, it's worth it.
BTW, my TW40X81 is about $2000US, so it's really not that expensive. And, it blows away ANY conventional set I have ever seen, including the vaunted Sony XBR400 series.