Is it painful to install?
Honestly, yes. It's not fun when someone slices your skin open and then lifts your skin to create a pocket, even with lidocaine injections. But usually it's 10 teeth-gnashing minutes and then it's over.
That's for big planar implants of course. The smaller ones are glass tubes the size of a grain of rice and injected under the skin, exactly like chipping a pet. It's 10 seconds and then it's done. But those glass implants have reduced performances, especially for things like doing cryptography on the chip, which requires more power.
Is it uncomfortable to wear?
No. I've had mine for years and I honestly only remember they're there when I touch them.
Where do have it?
My Flex is in my right wrist. I have other implants (11 in total) in other locations. Mostly in my hands.
What other things is it useful for?
The Flex is used for cryptographic purposes (TOTP and computing hashes, the main purpose of the latter being those encrypted volumes). It's also used as a token to unlock my banking app on my phone.
All my implants are used for many things. I guess the best way to show them to you is to point you to this post I wrote a year and a half ago, in which I gathered all the videos I made to show their use in one post.