I've bought Pebble time steel smartwatches with the idea, that I'll try to program them. See 📂The Most Personal Device experiment for details. This didn't go well for several reasons:
Pebble got bought by FitBit, and then they discarded all online services for Pebble programming. This means that cloud IDE and iPhone app suddenly wasn't available for programming purposes. iPhone app still exists, but it can't be easily used to program the watches.
Project Rebble appeared, but never got significant traction. You can use it to hack the iPhone app to allow you upload apps to your watches, and talk with them via Bluetooth LE, but its kinda broken, pairing often fails and it is altogether a horrible solution.
Open source alternatives did not delivered
There exists a project called FreeRTOS-Pebble, where people are trying to create new firmware for the pebbles. It's great project, but development is slow, and core features of the watches are not supported.
The biggest problem is, that it's really hard to use. You have to download and compile a bunch of C stuff manually, and go to discord to download bitstream for FPGA display driver and generally do all kinds of stuff, that you rather wouldn't.
I've tried to build a virtual machine in VirtualBox with the build environment several times, and I think I succeeded on last try, but I've spent maybe ten hours trying to make it work and that sucked out all my enthusiasm for it.
When it works, you should be able to create your own apps using C programming language. But you are developing for the dead platform, on almost dead opensource project.
This is one of those things, that I don't get. There is plenty of space inside the case, and they could easily make it bigger to support bigger battery. See the beefy and absurdly thick steel case?
With the battery that was used, I only got something like three days of battery life. That doesn't sound bad, but it was.
Failing flex cable
And then the flex cable, that is used to deliver information to the display, failed. It didn't stopped working, but began to show random patterns from time to time. To say that this was annoying is understatement.
When I've tried to repair the issue, I've accidentally destroyed the cable and that was it. Pebble experiment finally ended.
Was it worth it?
I've bought them in the middle of 2019 from someone on eBay, so it didn't cost me much (something like 50€) and it was an interesting learning experience.
It shown me, that software matters, and everything linked with cloud is dead in the long term. And also I've used them for more than a year as regular smartwatches.