Yes it's a good investment since youre cutting down on the wires since hdmi is an audio/video cable and also the best you can get but it's also expensive. If you have a receiver and a tv thats all you would need but then theres the optional surround sound, elite 360, cable, dvd player etc...
Also if you have the tv that supports 1080p then hdmi is worth it especially for movies and tv but gaming it's not all that hyped up as it is unless youre sitting back that way you dont see all the flaws in the game.
Just to add on, here my future setup with hdmi and such: Samsung 32" LCD with all the bells n whistles, surround sound with hdmi and decent speakers, Elite 360
^All of that should range at a price around $1800-2700 based on retail pricing. Now you know why I say it's expensive. I'd say my home theater plans are good. Whats your setup mr.invader if I may ask?