There are a number of us that still have the original Horizons. I have been using mine for the past 4 years and really have no immediate plans to upgrade.
Here are a couple of things that the older units don't do. (Maybe more)
1) No display of PDF approach charts. (I use an Android tablet)
2) No interfacing with the android app from GRT.
3) Due to the screen pixel size, no graphical engine data on the bottom of the PFD screen as is available with the newer models.
4) No video display from external camera.
5) Newer models have more data on the PFD screen (Like traffic in 3D I think)
From a positive standpoint:
1) Same robust AHRS
2) Same autopilot
3) Same interconnectivity with all the navigators.
4) Interconnects to ADSB fine.
5) You own it........
There is a screen shot of the WS on this page. Click it to enlarge
In my view, it is a personal choice on whether you upgrade now or not. The ones you have will give great service and are way better than anything you will find in a common spamcan rental....but, no they are not as capable as the more recent offerings from GRT.
Good luck with your decision.
Gah!, Katie beat me!