You can use the smart keyboard in your lap, but it is far from a laptop like experience. You only have one angle and the screen is top heavy compared to the keyboard. Your legs need to be almost perfectly level for it to be even half way stable and comfortable.
I almost never do it. If I can't find a flat surface to work with I take the keyboard off and use the touch keyboard, all hunched up over the device on my lap, or holding it and typing one handed. Not something you want to do for long.
But I also have the iPhone 7 plus. As long as there is not a lot of typing involved, or I can get by with the mosty good dictation, then I'm likely to use it when a table or other surface is not available.
Yesterday I was in Best Buy (don't live near Apple Store) and went to the Apple section to see if they had a keyboard case to try. They had a 9.7 with one installed and I tried it--didn't mind typing on it (and since i touch type found that was decent) but ran into the space key issue I've read about various places--that seems to occur after using the keyboard for awhile. I'm sure this one has taken lots of abuse but it still made me cautious.
For the time being I decided to stick with my logitech BT keyboard which pairs instantly on turning on. Still doesn't work for the lap but if I slow down a bit with the Zugu case that allows me to set any angle and is very stable lap typing works fine. I may at some point buy the Apple Magic keyboard because the Logitech (which i got on a good deal) has less comfortable round keys anf I sometimes type quite a lot. My old Apple keyboard just got exceptionally cranky about pairing--sometimes refusing.