The best advice I've heard is to buy the movie in advance and let it queue up. Not bad advice, but I'm just frustrated because my XBOX doesn't lag when watching movies and when I order a movie directly from my cable provider, it doesn't lag either.
I can't speak to the Xbox experience, since I've never used it but I can say comparing downloading and watching a Hi Def movie on
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As to the Xbox experience, are the movies you're downloading also Hi Def? If not, Standard Def movies will download and be ready to play must sooner than a Hi Def file and will not require as much buffering.
I suggest if you slightly modify your expectations, and your renting habits you'll be happier with the product.
Regards,
Michael