Pros
First thing to mention: Telemetry is no longer around, but the team still exists and is working on new projects in the advertising space. Now go by the names "Plan Blue", "Electric Ads" and "Rules eXchange" If you wanted to make an impact this place was amazing. Unlike other companies I have worked in nothing was siloed, and everyone was encouraged to get into everything. There were a few de facto head developers for certain bits of software who you were expected to consult if you started in messing around in their patch, but there was a real culture of "getting sh!t done" that, once you learnt the ropes.
Cons
There were a lot, but I think they all boiled down to this: The flat management structure resulted in very little barrier between the demands of customers and upper management and the lower levels of employees. This was very disconcerting to people who had come from bigger companies, and even people who had come from other startups where the founders had to answer to the VCs or other parties funding the venture (Telemetry was entirely funded organically by its founders). At times I wondered if some external funding, and the need for accountability it would have brought, might have been good for the company. The company tired to avoid this being an issue by only hiring people that wanted to "take over the world", but this resulted in a lot of big personalities hitting up against each other a lot. Their development methodology was based on breaking the parts of any new system up into parts that could be built by a single person and creating clean simple interfaces between them. Although this worked well it means you did not get the experience working using standard ways, like by using scrum.