I have considered many times.
Last time I was just about to pull the trigger, was when I upgraded from Rebel to 40D. If Nikon would have had a 17-55 with IS, equivalent or better than the Canon 17-55 IS, I would have switched.
When the 7D came out, I preordered .. first time in my life and have not regretted it. While my itch for FFis still very mild right now, I will eventually get a FF body. For that I find very little fail with the 5D 3. It has enough resoultion and speed for my phtographic interests. Overall, it is just about everything the 5D 2 should have been from the start. It is just the pricing which is really off. I could afford it, but refuse to give Canon more money than a better offer from Nikon would cost. As I am in no hurry, I will wait and see how long it takes until the 5D III costs less than the D800 (the day will come, maybe sooner than expected).
As an amateur, who does not shoot on a daily basis, I do not want gear from 2 brands in parallel. I am glad if I master one brand and its intricacies! I want an APS-C and a FF body from the same maker, one user interface, one set of lenses and flashes. I was looking forward to radio controlled ETTL and Canon has delivered and pulled ahead of Nikon on that one. Again, introductory price is ridiculous, but I'll also wait on this one.
In terms of lenses Canon has got everything I want, except a f/2.8 UW-zoom [eg. 14-24] matching Nikon.