I wouldn't, simply because the next generation is around the corner.

Let's say that I did purchase one, however.  At the current network difficulty and BTC exchange rate, that S5 would expect to make ~$3.25 a day before power costs.  ROI would be ~123 days.  Using a very conservative network difficulty adjustment value of 2%, we're up to ~136 days.  That right there should have convinced me not to purchase one in the first place... and I assumed free power in my numbers.