It probably is worth it, but I'd offer a word of warning. I came to the Firefly party pretty late, so had several years of people on forums going on about how brilliant it was. As a result I watched it with very high expectations and was, I'm afraid to say, dissapointed.
It is enjoyable, but it's nowhere near to being the best series evah!!!! that some people claim. It has a great cast, but the cast is also top heavy (thankfully sorted somewhat in the film) it has some good episodes, but too often any danger the cast are put in is resolved very easily, so there's limited dramatic tension at times.
I'm not trying to put you off, it is enjoyable, especially for the cast interactions, and I love the film, just suggesting that if you buy it try to come at it with an open mind and don't expect it to be brilliant perfection.