I'm a developer of the game so obviously I'm going to say it is worth it but even still, I truly believe its a much better game with Entrenchment. We've also combed through countless feedback from players and the concensus seems to be that it is a much better game.
Here is a short list of new features: http://forums.sinsofasolarempire.com/342094
Here is a overview of some of the new units: http://forums.sinsofasolarempire.com/342096
Next, keep in mind that all the most popular mods are now Entrenchment only. This is partially because with Entrenchment we were able to add a host of new modding capabilities that most modders like to take advantage of.
Finally, if you want to play a lot of multiplayer on Ironclad Online, most games are being played by Entrenchment players so it'll be easier to get in a game if you also have Entrenchment.
However, should you decide not to try Entrenchment, don't worry we are still upgrading and adding content to the original Sins. We just released a major update last week (1.18) and a hotfix this week (1.181). And most importantly, I still think the original is quite fun!