Hi PietStevens, I went from a Clie UX50 to an HTC TyTN before buying the Touch HD. The TyTN was a step back from the Clie in terms of resolution, so I was on the lookout for as large / hires a screen as possible. To tell you the truth, I was waiting for the SonyEricsson Xperia to come out, which had the same form factor as the TyTN. I came across the Touch HD when reading the reviews for the Xperia, the keyboard of which didn't impressive reviewers. I decided to go for the HTC based on screen size and resolution, after checking the various virtual keyboards available.
My experience after 3 months : you get used to it, but you can't compare with a physical keyboard. One of the advantages of the virtual keyboard mechanism is it works in portrait mode. In fact, the whole device is geared to be used that way, and it really is more convenient than constantly switching and pulling the keyboard (TyTN / Xperia). There are also many options to choose from, such as built-in compact QWERTY with 2 letters / key, an iPhone-like freeware or the various Chinese IME. But that also means the keys are small and you don't get the physical feedback of a keypress. I don't have large fingers but I still make mistakes and find it somewhat frustrating. I also find that I hardly ever use the virtual keyboard in landscape mode, which doesn't seem to offer any real advantage.
On the whole, I have no regret, the device is amazing, the lack of configurable buttons and interface takes some getting used to, but there is no match for the screen (iPhone lovers notwithstanding), lack of keyboard makes for a thinner device and the feel is extremely nice, no regret whatsoever. But heavy text entry still will be easier with a keyboard, unless you can get used to T9 predictive input which I can't (beyond English I also write in French so that complicates things). No such difference for Chinese text entry though.