They're definitely worth it and a very good investment, it just depends how far you want to take it. If you have a huge collection of movies then you could go with
XBMC which is great software, I've been using it for a couple of years. It has a few built in scrapers to obtain movie information for your library etc.
You'll first need to decide on what you want it to look like (mATX or full tower) as there are tons of designs out there (some even with 7" touch screen displays!).

Then you can either install a receiver so it has a dedicated remote control or use a mini keyboard, tv tuners are pretty slick nowadays.

Since you're planning on mostly streaming you could easily go with a mATX build, for libraries this isn't effective as there aren't many HDD bays in mATX cases.