You have done your research pretty well, since those are IMHO really the three best players in the <500e price range.
Both the NAD and the Cambridge have had some issues (NAD with the the transport, Cambridge with a buzzing transformer, but Cambridge updated their design a while back - so if I were buying Cambridge, I'd cough up the extra for a new piece instead of buying used.)
The H/K won the EISA award this year, but was criticized for it's somewhat cheap and plasticy finish - and I agree on that. OTOH, the display on it is really nice looking, I'd rank it as the top one in the three, followed by NAD and third by Cambridge (Does cambridge still use a different display for it's black models? I've only seen the "black on silver" one used in the silvers). For general looks, it's a matter of taste, with the Cambridge being all-metal and the two others using plastic (NAD for the faceplate).
Soundwise, I've heard the Cambridge briefly and own the 521Bee. According to press reviews, the Cambridge would be the brightest and the H/K darkest with the NAD in the middle. The NAD as a nice, fairly rich sound that I like. Probably slightly rolled-off at the top, though.
For those with multiple digital sources (ie. a computer) the H/K has both an optical and a coaxial digital input - if I were building my budget delight setup, that's almost definitely the one I would go with today.
All in all, I'd say it's very hard to pick a clean favourite among the three. They all have definite qualities which I hope to have illustrated to some level. I think it boils down to personal musical preference, connectivity and the fact the appearance-wise neither the NAD or H/K mix that well with other equipment (The H/K maybe better, but a question of taste).
Personally, (I'm pulling the traditional head-fi "If you spend double, you'll get something slightly better) I'm waiting for the Cambridge 740c to come out, as it would seem to combine the best parts of the H/K (AD Blackfin based asynchronous re/oversampling and external digital inputs) with the Cambridge minimalist appearance and good review tradition.
Hope I was of some help at least,
/JF