At the current moment, the iPhone 5s isn't worth $650, in my estimation.
However, we're in mid to late cycle, and unless something unforeseen happens, there'll be iPhone 6 by Sept, at the latest. This is why I don't think it's worth it, right now.
I've been giving this lots of thought, especially with ATT's new plans, where buying the phone outright is an option. Assuming there's no subsidy, I'd pay $650 for a new generation iPhone at launch, but not after, and not the mid-cycle upgrades.
It's difficult to be fully rational (others would call it fanboism), and I can't explain it, but knowing that I will overpay for an iPhone (just like it much better than Android and iPhone's build quality) means I'm sure I will on iPhone 6. Actually, I'm positive I will pre-order it, regardless if it's subsidized or not. Bear in mind, this is even without seeing it yet!
However, I wouldn't after launch, mid cycle, or an in-between (s) or (c) upgrades.