I think you've hit the nail on the head. We are an existing TCM shop that is evaluating TPM. While there are some very nice features, because TPM is a radical departure from the TCM line, there are things that are missing. We've put a good deal of development into integrating TCM with our change control process, which includes Rational ( another IBM aquisition ). TPM does not support the same command line interface as TCM, so much of our automation does not carry over.
We're also concerned about how well TPM will scale. We support a relatively large installation, and from the looks of it, TPM is more suitable for small to medium sites.
I'd really be curious to know if any larger customers have adopted TPM.