Wanted to add: in my case, SQ improvement was impressive, but I have a 2000LX which has the older single-DIN unit (not sure who makes it... it's built in Malaysia just like my Kenwood...).
My ex-wife has a 2003 (drove it myself for 3 years) which has the same double-DIN unit as you do (which I believe has similar power ratings as the Kenwood). This unit, the one you have, isn't bad at all, it sounds very good for what it is. What you gain with an aftermarket HU is mostly MP3 and USB capability (a major for me) + more tweaking capability (hi-pass filters, parametric EQ, etc) and genuine RCA outs (3 sets, 4V) for easier addition of amplifiers (and inboard crossovers). The Kenwood can push more bass to the speakers though, but they won't handle it... If you change the speakers however, then the HU will shine.
If I had a 2001+ model, I'd probably add a sub before changing the HU.
Of course, now that I'm used to the USB port and MP3 (and AAC!) CDs, I wouldn't go back.
Since it reads AAC files, I can simply rip my CDs with iTunes then put anywhere from 5 to 15 albums on the same CD-R. Only problem is, when reading compressed files, it pauses for a second or two between each song, so when you're listening to an album with songs that "spill over" each other, it's kind of annoying. Like Pink Floyd's The Wall (where it seems every song it that way). With regular audio CDs this doesn't happen.
Also, the USB port will power most MP3 players, just not USB hard drivers.
And one thing not advertised: it will play files on an iPod even without the adapter (well it does on my iPod Shuffle, didn't try with the Nano or the Video). The songs are stored as files on the iPod, they are simply hidden (the iPod is really just a big USB drive with an MP3 player built-in). The eXcelon does see them and play them (and display song info), and it does charge your battery at the same time. And the sound quality is better than using the iPod adapter (which uses the aux input). Only problem is it won't play files bought from the iTunes store. I don't care, I'd never pay for a compressed version of a song. If I'm gonna pay, I want the CD. Like I said, I only tested this with the iPod shuffle, so I cannot guarantee results with a Nano or Video iPod.
The display dims when you turn on the lights, but the blue rim around the volume knob doesn't and it is BRIGHT.
I'll try to get a picture. My 2000 had a single-DIN unit, so it already had what people here call the "MSP Pocket", so installation was very easy and it doesn't look too ghetto. Basically looks like a MP3 or MSP, except the newer Kenwood HUs have a classier look (less "pimp", more black). Maybe you could get the pocket out of a 1999-2000 at a junkyard... You need the little piece in the middle as well that holds the radio in the top half ot the opening. I found it easier to install the radio first, plug everything from under, then put the pocket in. I used a reverse harness which took maybe 15 minutes to connect to the Kenwood harness using butt connectors (although if I had to redo it, I'd solder the wires and use the shrink-wrap stuff that you heat). If a speaker wire comes appart and touch the frame, the HU is dead, so make sure your connections are solid.
A lot of people here hate Kenwood mostly because the stock unit on the MP3 and MSP would get stuck with the face "semi-open" (they have motorized face plates). The X491 doesn't have a moving face plate, so no risk there. And since it's an eXcelon, the warranty is 2 years instead of 1.
