The A7 has a retail price of £1300, while the A7R will come with a £1600 price tag. Both models will be released in late November.

It's a good question. The mirrorless systems are maturing nicely, and with full frame becoming a reality it's looking very interesting indeed.

As someone with money invested in the SLR format, and not looking to replace anything any time soon, I'm watching the rise of mirrorless cameras with some interest. Personally, I like the weight and feel of a decent camera body. It strikes me a micro-four-thirds or mirrorless full frame body gets pretty close to SLR size once you've factored in viewfinders and lenses, and I worry about grip. I like something I can hold firmly, and I like the optical through-the-lens system despite wearing glasses that get in the way.
There are advantages and disadvantages to both, and the distinction of interchangeable lenses is now irrelevant. When the time comes to consider replacing my EOS 7D, I may well consider trading in for a switch to a different format. I know one well-established professional who has already made that decision (though, to be fair, he's retiring from professional work and wanted a system he could still carry without needing trollies and assistants!). Whether mirrorless will eventually supplant DSLR I can't say. Only time will tell, I suppose.