Visited a number of brick and mortar stores to take a look at what they have and the chain stores (Spence's, Lugaro, Ben Moss, Michael Hill) sure carry a lot of crap - low quality diamonds, poorly constructed rings and boring designs. Were it not for the fact that the g/f and I are looking for the right ring and wanted to try different styles I would have walked out of all of them in a couple minutes. Birks was the exception - very limited selection but clearly high quality stuff, those diamonds pop and those rings are beautifully made and those prices reflect it.
A couple of the family run shops I went into were pretty high quality and had designs that were far more interesting (even for their solitaires). They offer custom work for only a bit more than the cost of an existing ring and were able to provide much better advice.
That said, the online shops (Blue Nile and Brilliant Earth) have the best prices and offer quality that compares well with Tiffany and Birks. If you know what you want and they have it then I don't see why you wouldn't buy it from them.
In terms of spending 10K on a ring - gosh, best to you and those who want to do that. I'm comfortably banking 6 figures and don't plan on spending more than 5K (a figure the missus agrees upon). On the flipside I have friends who make half as much as I do who have spent 15K on a ring.
It just seems like an awful lot of money to spend on an object like that - I could get a set of Ohlin coilovers and a Flying Miata SC kit for less than that!