Two people, prototype in 2 months (CTO of BugSense here)
How close do you work with your clients and how much is your system changing in response to feedback?
What drawbacks (if any) the current tech stack choices had so far on your?
Our clients were involved since day 0 (which I think is the key of our success btw). The first prototype (built in two days) was in Erlang using Mnesia.
The drawbacks are: + not having lots of engineers knowing C and Erlang + Outsiders having problem with LISP (that's why we implemented PIG)