Exactly.
It's not a perfect solution - nothing ever is, there always have to be compromises in the real world.
In relation to the Cambridge area busway, the heavy rail option would almost certainly have been the worst one due to high operating costs in relation to service frequency and no ability to directly serve the central part of Cambridge. It would likely have required a complete rebuilding to modern standards of what was only ever a single-track branch line (and of which the extension earthworks to Huntingdon station from St.Ives had been partly obliterated - not least by the current A14 road).
In the current timetable, off-peak Mon-Sat there are 8 buses per hour between St.Ives and Cambridge city centre in each direction - and that is higher frequency than when it opened due to it's popularity (it carried 3.5 million passengers in the year up to August 2014). Once the 'Cambridge North' railway station opens (to which the busway will be extended) I'd expect those passenger numbers to increase noticeably because people will be able to go directly from (fast) bus from St.Ives etc. to train.
It seems to be mostly outsiders who criticise it - the locals just happily use it
But the question should be, what have you gained by destroying the railway option, that couldn't have been achieved by providing the necessary busways within the urban area of Cambridge (where most of the congestion is likely to occur) and making the necessary improvements to the existing road (presuming the road between the two wasn't one continual traffic jam). You could have had a less expensive, but fit for purpose bus network for local journeys and still maintained the option of a direct train service for longer distance by extending some London trains to St Ives.