I've been playing it and loving it. Granted, it is really short, I've only been doing one episode a night because it's so short, just to try and stretch it out a bit. Still, I think I'll probably play it through at least one more time after I beat it. You could undoubtedly beat it (probably even multiple times) over one rental period. I knew this when I bought it, but I wanted to buy it because Remedy (the studio that made it) pretty much bet the bank on it, so if they don't sell enough copies they'll go under, and I think that'd be really unfortunate because it's an excellent game, I would even go so far as to say that it is the very best example of the blending of storytelling and gameplay elements that I have ever seen.
So, if you want to support a good company and a good game, go ahead and buy it. If you couldn't care less if Remedy ever puts out another game, then go ahead and save yourself the money by renting it.