I use the M42 a lot as I live in Solihull
Mostly when it's busy using the hard shoulder is OK (even allowing for vans and BMWs) the big drawback is the speed limit settings
It's supposed to be automatic and based on traffic flow, etc. but nowdays it hardly ever seems to be set at 70
On a relatively quiet Sunday where you have been happily bowling along at 70 (give or take!) as soon as you get to the controlled section the limit is 60 or maybe even 50. The hard should is rarely open these times open though!
I've been on there a few times and alternate signs are 50 60 50 60. Bearing in mind how close some of these gantries are and the fact that they have M25 type speed cameras installed, I seem to spend a lot of time looking at my speedo.
When the road is not busy this artificial limit causes bunching and I have a seen a few near misses at junctions when it has been difficult to move over as the traffic is travelling closer together and at roughly the same speed.
The other bit about this "experiment" that is odd is they have removed the countdown markers from some of the junctions. This means that at the last minute, when strangers realise the junction is upon them, there is a fair bit of last minute lane changing. I wrote to the DfT about this and they said it was done to avoid any confusion approaching the new refuges!? Hmm.
As to the worry of no hard shoulder for breakdowns - how often do we see cars there anyway? Modern cars are (mostly) very reliable and a breakdon is an infrequent occurrence.
Personally, the jury's out, but I'll bet the speed limits and cameras must be raising enough revenue to pay for the work.
Cheers
Mike