The chain is fine, no less durable than 9 speed (for using it for over a year).

It makes double or single rings more accessible, personally I like it for that reason alone. I wouldn’t personally go 3×10, but then I wouldn’t go back to 3×9 either.

For short laps I’d agree that 2*10 makes a lot of sense, but when you are tired at the end of a long ride, and you need to get up big hill, I need a granny ring and I’m not ashamed to admit it.

Like it or not, it is the future. In 3 years time decent quality 9spd kit will start to become harder to get hold of and the big Ss will have got one over us again. I am not sold on the advantage of the dinner plate rear sprocket but a closer set of ratios does appeal. Not enough for me to splash out right now though.

Am I right in thinking that you can replace a 9 speed cassette with a 10 speed onto the same free-hub?

No benefit that I can see other than you can have smaller jumps between the different cogs. The more gears the less difference there is by adding another one in this respect though.

I was also under the impression that gears should not be used across the whole rear spread from one of the front rings. Obvious for the big and little ring but I did read you were only meant to use the middle range of the read for the middle front ring as well (Brian Loppes book before you ask). So maybe 2X10 is not so good but I always use my gears the wrong way so who cares.

It is just another gear so I would not go out of my way to change. If changing your whole drive train anyway way not? Inless you can get 9 speed cheaper.

Doubles have been available for age eg Middleburn XC Duo must be 15 years old by now? Cannondale used to ship all their XC bikes with their CX-2 chainset eg the Scalpels etc.

10 speed = 9 too many.

Surprised no one else has said that yet.

Correct, but the wider cassettes get the more feasible they become as you get a wider spread of gears. A double with a 12-28 7 speed block requires pretty good legs, with an 11-36 less so!