The more I think about it, the less I like the Joy-Con controllers. As has been discussed ad nauseum, they are too small and the buttons arranged too awkwardly to be used as single controllers. Also, that grip controller looks rather strange, and I think I would just use the pro (yes I plan to buy regardless).
Now the thing that bothers me is a lot of Nintendo fans and the writers of Nintendo sites seem to agree, and write off the concerns by saying they simply won't use these features.
However, this is the essence of the hardware design. Think of the design compromises they had to make to pay for and make room for 3 batteries and wireless radios, and two side ports, just so they slide off for no good reason. Also, with extra moving parts, it becomes less durable, and opens up the possibility we would need to buy more peripherals down the line. In other words, if you don't plan to use the Joy-Cons as stand alone controllers, the fact that they are removable makes the console worse, not better.
I feel it would have been better if the switch simply had the sides built in and come bundled with the pro. The cost of the pro would easily be off set by the decrease in costs of two batteries, wireless radios, and side ports, plus the cost of the grip center piece.
Lifetime Sales Prediction - 6/29/2013
Wii U - 38 million
XBOX One - 88 million
Playstation 4 - 145 million