I have a HX-D2... I've thought about upgrading (to Pioneer P90 combo) and even downgrading it (Pioneer P80 and simplified system) throughout the time I've had it (about a year now) but at the end of the day it's still in my car, and it probably will be until it either dies, or Clarion releases a successor.
I have briefly used HX-D1, F1 Status, Pioneer P90 and Eclipse CD8455/CD7000 (amoung others) and yet to this day I still consider it the best looking and best built head unit I've yet seen.
As mentioned the remote control is about as useful as a square wheel (it's about as responsive as a rock) but outside of this it's tunability is up there with pretty much anything else i've seen.
The only things it lacks against it's more expensive competition (F1, P90) are crossover slopes (HXD2 is limited to 6db/12db/18db) and 31 band EQ (5 band parametric only) - however that doesn't necessarilly tell the full story.
The sheer adjustability of the Parametric EQ in terms of gain (-12dB to+12dB in 0.5dB increments), frequency (1/3 octave) and Q (0.5 to 12.0) is the most comprehensive parametric EQ I've ever seen on a head unit, and if you have a system that is good overall with just a couple of response problems, it may actually prove even more useful then a full on 31 band graphic (part of the reson I decided not to P90). On top of this, unlike some other units (such as some Alpine's) the unit doesn't try to outsmart you by limiting how close you can position bands... you have pretty much completely freedom to place your bands wherever the hell you want, and even overlap them all if you really want to. It also gives you individual L/R control, allowing you to use L/R independant EQ if necessary.
Aside from the lack of a 24dB/30dB slope, the 4way active crossovers give you tons of control. They offer high and low pass filters on every channel (except tweeter, which has low pass only) and give you level adjustability from 0dB down to -24dB. Every channel has the option of phase reversal (0/180) and all channels can be individually muted which seems petty... but has proven to be one of the most useful features I've ever had on a head unit.
Finally, you have time alignment adjustment from 0cm up to 512cm uniquely adjustable on every channel down to about 3mm accuracy.
To assist with tuning further, there are three custom memory presets as well as a default preset (if you need one to get you out of trouble). Unlike some Alpine units I've used, you don't need to seperately store time alingment settings, EQ settings, etc. You select one memory and it saves ALL settings in one go.
With the exception of a single CD (it seems to hate chesky) the operation of the unit is flawless - perfectly smooth and quiet, with no software errors, crashes, read errors - nothing. Stunningly reliable, probably largely due to it's simplicity.
Finally, while the menu initially seems a little overwealming (due to lack of a GUI), once you get used to it it's actually one of the simplest user interfaces around. Every function does exactly as it is labelled, with no pointless, ambiguous graphics or bs to confuse. The pure text display is perfectly clear to see in all lighting conditions (unlike a lot of fancy looking LED displays), and navigation of the menu system is pretty much instant, with no lag or delays. It ain't fancy, but when it comes to fundamentals, its the best user interface i've used yet because it's WYSIWYG (What you see is what you get).
There are no pointless frills with this head unit - it doesnt try to dazzle you with flashing lights, animations, mario-esque sound effects or fancy gimmicky MP3 interfaces...the more you use this head unit the more you realise that it's designed purely for sound, and nothing more. Everything that is in there is there for a reason, and I absolutely love that about it.
I very seriously considered upgrading to a Pioneer P90, but over time I have seriously began wondering if it will actually bring me any improvement...in a brilliantly installed system, with the right components, the 5 band Parametric is probably more useful then a 31 band graphic, and I think i'd miss the styling, simplicity and obsessive adjustability of the Clarion more then I'd welcome a 24dB crossover slope, which is essentially the only thing the Pioneer would offer me above it.
Compared against a P90, even if money were no object, there is a good chance i'd choose the Clarion.
If someone offered me a CD700 II for my Clarion I wouldn't even need to give it any thought - the answer would be "forget it". Even if you do have a seperate 31 band EQ, unless you install is nigh on perfect (I've yet to see one that is), and you are a passive fanboy who will never in your life consider running active, the time aligment and crossover features on the Clarion are far too valuable to throw away, and it's sound quality is as dead accurate as I've heard from a car head unit.
If you go the Nak then given the price point, my guess is you'd be paying 25% for the sound quality, 5% for featrures, 20% for the name, and 50% for the wank factor.
Just my 2c