In my company we use upstart on our product, an embedded system, mainly because at that time it was the most suitable init system on Debian: It starts and stops our daemons, it handles dependencies between them, it does restart crashed ones - that's what we needed. Boot times were not relevant in the decision at all.
Replacing sysv-init with upstart was easy and straightforward, so I assume we will stay with upstart when we in some distant day upgrade from Debian 6 to 7.
I'm saying nothing here against systemd, which at the time just was not available in Debian, if I remember correctly. We are totally agnostic in respect to the init system, as long as it does what we want, i.e. is not sysv-init.