I've read some really good screeds against X over the years, but ultimately it all boils down to a very simple question: does it work, nor not?
Overall, I'd say that it has held up rather well over the years. You can do a lot of really cool things with X, working through tunnels (ssh of course). I used to occasionally open up Firefox (or whatever it was called that week) remotely using SSH to test how a website looked from outside a corporate firewall. It was cool, secure and useful. Biggest drawback was speed, but that wasn't my primary concern at the time. X still has speed issues but there is a lot you can do to optimize things. I still think it is cool that I can open up a gui application on a computer on the other side of the planet, and aside from responsiveness, not have any real issues at all.
As a developer, I like X because it's just another application. If the X server gets wedged (a very rare occurrence nowadays), everything else just keeps on ticking along. Simply kill the Xorg process and all is well.