My Nokia 6 TA-1021 works at the same speed as usual with Oreo beta.
Overall stability is fine, but some new Android features in for example the battery monitor and Bluetooth are not yet finished.
Most of the third-party apps I use work as usual in the beta.
I'm not sure participating in the beta program is worth my time? - I miss a changelog and a bug tracker and feedback regarding known issues in this forum.