Well I figured I've had enough play time with the Kenwood DPX503 HU to give it a decent review. I'm not the best writer and definitely not the most knowledgable guy on here, so bare with me.

I chose this HU because I wanted a HU that would fit in the double din slot and look rather factory while having decent processing capabilities. I've had a few double din screens but they really stood out and I wanted to keep my setup rather simple this time around. The Pioneer double din was a little too flashy for my dash and it would've looked out of place, I also wanted usb and aux inputs to be in the front so I could have quick access to my zune or thumb drive. There's a double din Clarion HU as well, the reason I didn't go with that was because it looked cheap to me and the features of the Kenwood seemed much better.

I picked up the Kenwood unit from ebay for around $215 shipped brand new. It has a ton of features and sound options:

Full Ipod control through usb input. I didn't have an Ipod at the time so I can't say how well it worked with the unit.

Front USB and Aux inputs. These are very convenient for those long trips. I have a 4GB thumb drive and it loads up and begins to play in about 2 seconds.

Sound Processing. Isn't too helpful for the active crowd w/ only having 2 sets of preouts and you can only HP the fronts and rears to 40hz/60hz/80/100hz/120hz/150hz/180hz/220hz/or pass through or you have the option to LP the rear outputs from 50hz/60/80/100/120hz for a sub and it will give you individual subwoofer level control and phase control.

It has an option for you to turn off the built in amplifier. You can pick what type of vehicle you're in (anything from compact car to cargo van) to help with cabin gain. It has a switchable speaker size setting although I don't really know how that alters the sound. It has a standard EQ that allows adjustments for bass, midrange, and treble.
One of the better options for the sound is the T/A. It has 2 settings for this, DTA, which controls front, rear or sub speakers or you can fine tune it manually with another T/A setting that allows you to control each individual speaker in .07ft = 2.13cm increments. DTA I would guess has a better use for mids and tweeters close to eachother or coaxials? You can save your settings in a memory option but it only allows 1 saved preset. And finally, it has dual zone capabilities for you that have a dvd player in the back.

Integration/Illumination. The unit has 10 standard colors you can choose from or a user option where you can fix the red, green, blue and tailor the color scheme to your liking. You can change the overall color of the HU or each individual piece that lights up. For example, you can make 1 knob green, the other blue, the display red, etc. You can also make it dance to the music.

Misc. Has a 4 digit security code and has a few different display settings to choose from. The displays are rather simple but effective in showing just about everything you need to know. It does have Bluetooth capability but you have to pay extra for the adapter.

One of the main issues I had with this unit were that it only had 2 sets of 2.5v preouts. It really would have been nice if it had another set even if the price was raised a bit.
The illumination for the HU is another thing that has issues, it's just not bright enough and gets washed out during the day (even with tinted front windows). You can change colors which helps because there are colors that show up better than others and you can also cycle between the lettering showing color with a black background or the other way around. The illumination also allows for a saved preset.
The xovers obviously aren't the best but I suppose it's enough for only having 2 preouts. The EQ also doesn't allow for much tuning.
Scrolling through the options is a pain at first but you sorta get the hang of it.
The manual is pretty confusing and I still haven't found some of the options that the unit apparently has.
The Remote is horrible! You can't REW or FF with the remote? What the hell is up with that? Another problem is that there isn't a button for you to get to the menus. You have to press function on the HU itself and then you can scroll through the menu from the remote. Overall the remote is pretty basic and doesn't allow you to control a lot of the functions the HU has. Vol, ATT, play/pause and the basics are on there though.
I think the only other thing I find a little annoying with the unit (which goes back to the remote) is that there's no on and off button on the remote itself. Not a problem for most but I have my HU running it's power from my battery due to a weird issue I had with the stock power harness. I guess I just don't like touching my HU's too often and because of the crappy remote I have to do it a lot.

Overall a nice unit for the price and it looks to be one of the better stand alone double din units out there. Not up to the standards of a lot of people here but for a simple setup it's very affective. Sound quality is good, not as laid back as my Pioneer 800prs and not as in your face as an Alpine. I'd put it right in the middle of those two. I'm running a 4way active setup but I also have a simple Coustic active xover to help with that.

So, that's pretty much it. If you have any questions or comments please let me know. Here's some more info on the unit and some more reviews:
Kenwood DPX503 CD receiver at Crutchfield.com