I have the Desire and was lucky enough to get a ridiculous deal on a Desire HD, so now I have both. Before I got the HD my thinking was the upgrade just isn't worth it, the only reason why I did it was because of this deal. Anyways, here's my two cents:
The screen- I thought the DHD would be too big, but I've actually fallen in love with it. Its also much more clear and precise than the regular desire screen for some reason.
The battery- its very comparable to the regular desire, although slightly better, again I was surprised at this. With equal usage I got slightly longer results from the DHD.
Call quality- for the DHD has the desire beat on this, it's much louder and more clear than my desire.
Those are the things that HD was an upgrade on for me. I have no lag issues or anything on my Desire, so the DHD doesn't have a noticeable speed upgrade for me. The thing really missing from the DHD for me is LED notifications. First of all, its hard to see, secondly I miss being able to assign different colours lol
The camera was not an upgrade imo. I took the lens cover off of my Desire and I actually think that its camera is slightly better than the HD.
Other than that they are pretty much the same. I will probably keep the DHD and sell the Desire, just because I really love the screen and the call quality is much better. So while I can't comment on the other phones, I don't really feel its worth upgrading, they are basically the same, unless you get a great deal. Don't get me wrong, they are all probably great phones, but they are just so similar it makes it a bit pointless.