I was very much against the CGB, and in construction it was a disaster that has blighted the name of busways badly.
However it also suffers unfairly from being compared against a hypothetical perfect railway rather than any realistic alternative to a busway. In operation it has generally been very successful (and IMHO should have been more so if operated sensibly), and I think in comparison with realistic alternatives it wins. It certainly wins if you compare the Busway what has happened to buses on the non-busway corridors around Cambridge, and if you compare it's value to public transport compared to other former railways around Cambridge. I am totally converted, and would love to see the Mildenhall and Haverhill lines converted to busways, and I would love to see Waterbeach / Ely and Cambourne busways. The success almost annoys me slightly because it it illogical (surely the main bus service to Huntingdon should be along the new A14, or even via the A428 that is still quicker?), but this is where we are and humans are like that.
A busway is not better than a perfect hypothetical railway. If a double track line with services through to Cambridge had been opened, and if it had a metro style frequency to Cambridge, and had a station at St Ives that was part of an integrated transport network had been built, then that would have been better. But none of that was or is at all realistic in the UK because that it not what we do (nor is it even what Cast Iron proposed). If that was realistic why is Newmarket saddled with a dreadful service? Why is Soham stuck with an even more dreadful service (despite a better service being a 2018 franchise committment) heading two wrong directions? What would we have done with a St Ives railway in the UK? We'd have opened a route with short platforms and insufficient double track with something like an hourly service with two carriage diesel trains, that may or may not have terminated somewhere in the North of Cambridge. It would have had poorly thought out bus connections that would have been abandoned within 6m of the station opening (like indeed the N service that the Busway briefly hosted when Cambridge North opened!!!), it would have had greatly insufficient capacity and rather than increasing it we'd have talked about doing something until passengers resolved the overcrowding by giving up and stopping trying. That is the very best of how we might have run a railway in the UK, for of course the reality is that the Haverhill and Mildenhall lines continue to be neither busways nor railway lines, and East West Rail continues to have only vague prospects of reaching Cambridge.
More realistically, the busway service is better than what was there before and, by comparison, every other bus route around Cambridge. The bus service to St Ives expanded massively (even while Northstowe was being late), whereas services to Ely have been decimated, services to Royston have been more than cut in half, Stagecoach entirely gave up on Newmarket, Cambourne has more or less tred water despite massive expansion, and Haverhill is much as it always was.
Naturally, the busway could have been better in conception. If cost cutting hadn't reduced the southern section to single deck only then it would likely be the main route for two park and ride services and services to Haverhill, and in doing so provide the real benefit of bypassing slow urban traffic rather than what the northern section does; bypassing countryside and dumping buses in town. It could also have been better in building, but then so could many railway lines and the Edinburgh tram!
And of course the busway could have been better in operation. Covid didn't help, and it's a shame we never saw the exciting April 2020 timetable that would have really tried out the benefits of the busway with regular services to Alconbury, Brampton, Somersham and Chatteris. It would have been nice to have found out whether they'd have worked.
The reaction to the fatalities on the southern section is something that baffles me. They happened because people left a pavement and fell onto a road in front of vehicles. I do not understand why the busway is being held to a higher standard than every road in the UK. On every road in the UK if you leave the safety of the pavement and land in front of the path of a bus it is likely, unfortunately, to at best hurt a lot. In most cases cyclists are even already on that road where they could come to that harm if they fall. I do not understand the decision that buses could not be alongside the pavement like they are in every other road and pavement situation in the UK. And I most certainly do not understand the decision that if the buses that gave the busway it's purpose and the pavement could not co-exist, it should be the buses that were banished. I guess that if pedestrians and cyclists had been banished to the alternative roads that statistics show are much more dangerous to them the "problem" would have been resolved much sooner. It seems to me that in life the busway has been cursed with the same comparision with railways as blighted it in birth.