I have V+ and I also have a 47" Full HD TV set, BluRay player and very expensive cabling whilst my nephew has an equally impressive 52" Full HD setup with Sky HD and what I can say with absolute conviction is that most of the HD content is not that great (on either)
The technical reasons are that a large proportion of HD content is broadcast with a resolution of 1280x720 rather than 1920x1080, which means that the TV (or other assigned hardware) is having to scale the image. Although, because of the maths involved, the scaling is much easier from 720p than from SD, it still does lose fidelity. In addition to this, the HD broadcasts are compressed, so even though they start off as HD some of the detail is lost. The few 1080i broadcasts I have seen do look a lot better, but still not anything like the quality of true HD on BluRay.
Don't take this the wrong way though, HD content is better than SD, just not THAT much better. Also, if you have a 720p TV then you will probably benefit more from the HD services available (But equally will not benefit as much from BluRay)
One thing I have noticed is that on Virgin, BBC1 is usually a reasonable picture, but if the same program is being shown on BBC HD then the picture on BBC1 is more highly compressed and practically unwatchable at times, so, for example, I would not recommend watching Strictly on BBC1 on Virgin if you don't have HD as the terrestrial picture may well be better... but the results show will be fine because it's not on HD!