I've got the Garmin Nuvi 885T - it's AWESOME on the road. It has found routes to a couple places I go regularly that I never thought of that save me time & distance...
I previously used Microsoft Streets and Trips a lot with a laptop and GPS bug -- that was a major PIA compared to this - with S&T, if you missed a turn, it would just complain that you're 'Off Route'. With the Nuvi, it just recalculates the route based on your current position (this came in handy once or twice in the ride down to JeepSkool). It *did* try to direct me over a closed bridge once down in Ohio, but that was on a back-country road and was the only problem I ran into so far, including driving around Orlando and Daytona Beach FL.
It's got built-in mp3 and audio book readers so I can just plug it into the AUX port of the radio and listen to a book on 'tape' (from audible.com) or some tunes when I'm traveling without having to mess with finding radio stations. The FM transmitter in it (and in all of the Garmins I read reviews on) is too weak to give you a good signal so you're better off with the direct connection. The advantage to using this over the CD player in your XK is that, if the GPS is directing you somewhere, it turns the volume down on your music and gives you the voice directions over it. If you just have the CD blasting, you might miss the voice directions.
It's also got a little tank icon for the vehicle (named 'The Commander' by Garmin) that's pretty cool, too
. They apparently won't do vehicle-specific icons to avoid being sued by the automakers.
Off road it works pretty well; the TOPO maps are available for it but even with the included road maps by default, it marks your path so you can easily find your way back if you get turned around.
I bought mine refurbished from buy.com - much less expensive than the new versions. Downside of that is that the battery will now only hold about 50% charge and needs replacing, but that's $30 and I saved much more than that.
It's not the best for geocaching - a new little hobby we're trying out that gets us into the XK more often
but it works well enough even for that. I think the 500 you're looking it is more geocaching friendly - I recall seeing it listed as so by Garmin, but can't recall the reasons it's better.
I've been trying out the MSN Direct sample subscription that came with it - it gives me real-time traffic alerts, lets me see where I can find gas local to my current location (and the PRICES!), weather, movie times, local events, and other stuff. I was really getting into that and then just learned that Microsoft is dropping that service in January 2012. Oh well.
Bottom line is that I really love my nuvi and even my wife, who is often dubious of my gadget purchases, thinks it's cool...