I had a prototype D4 to review, and I also got some freediving instructors to give it a go as well, to test that side of things. It replaces the Mosquito and fits better into the D series of computers.
It is well-made, easy to read and navigate and, as it has pretty much the same basic innards as the D6 and D9, there hopefully shouldn't be any gremlins appearing.
If you are thinking you might be looking to advance your diving and possibly end up doing accelerated deco, etc, then it might be worth saving up a bit and going for a D6, which gives you two nitrox mixes.
I've got both a D6 and a D9 and the only real difference is that the D9 comes with a transmitter and does a third nitrox mix - and it looks more 'bling, bling'. I actually prefer the look and size of the D6 as an everyday watch, the D9 is a bit clunky.
Mark