I used TF very early on, for versions 0.3 and 0.4. It was unpleasant at best, but very functional.

I've began using it again with 0.11, and with 0.12 on the horizon it's much easier to use than it ever has been in the past. It used to be very difficult to do one-off tasks with Terraform but I've began managing things piecemeal and the community around importing everything into existing TF things are great.

The statefile borking itself from time to time is still an issue, but no more than any other operational tool of this magnitude.

The biggest downside to Terraform currently (0.11) is that you need to get extremely creative in ugly ways to accomplish some tasks. This can been seen very specifically in the AWS Security Group official TF module, which I could not live without, but would not want to write/maintain.