I've been using the A431 through Kelston whenever I get the chance since the landslip caused it to be closed at the beginning of this year. It was closed to motor vehicles immediately, then about a week later closed to all road users, but a few weeks later a footpath alternative was constructed in an adjacent field, about 400 yards long, which cyclists can also use, to by-pass the actual landslip. It is limestone dust path, but does involve carrying the bike up steps to reach it.
The section of the A431 around the area of the landslip does give fantastic views over the river Avon countryside and also up into the Cotswolds above, and the round trip along this and then back along the Bristol & Bath Railway Path has been a wonderful almost car-free run from Bath, which very few people seem to have cottoned onto. I haven't yet tried it since the toll road opened on Friday, but the A431 is a narrow twisty main road, so even just 500 cars a day will make it quite a bit less pleasant (though still worth taking for the views).
I wouldn't want to try to use the toll road, as it involves a steep climb into the field and from what I've seen is very rough and bumpy, far worse than the alternative temporary footpath.