It's great if you've got time and you don't mind a wander. However, if I'm actually trying to get somewhere in an unfamilar area, I would never trust a cycle path. They always dump you in an unfamiliar place with no onward signage, and it's not usually because of vandalism. If it was only vandalism, they'd paint the directions on the paths...
I remarked somewhere else that the designers just fail to apply road principles. When you drive from Watford to Rugby, you do not look for signs to St Albans, followed by Harpenden, followed by Luton, followed by Ampthill, and so on, and you would not be impressed at being dumped in a playpark, however nice, somewhere near Milton Keynes, and left to find your own onward route.
There are no signs in Watford to Rugby, so in fact you look for signs to "Birmingham" or "Leicester" or "the North" for most of your journey, even though you have no intention of going as far as any of those places.
In the same way, the only feasible way to signpost a cycle path is to point to the next town. Whether you actually believe that anyone wants to ride that far has nothing whatever to do with it. It's a direction indicator, not a compulsion to go all the way there. I'm at a loss to explain why, instead, councils signpost only the next village or destination, when they know that doesn't work on the road*.
In practical terms, then, riding across Milton Keynes is impossible because signs only ever go to the next destination, some minor area of the town that you have never heard of, when they should go to Aylesbury, Bedford and Luton. If they did that, you'd have half a chance of getting across town to Phil Corley's bike shop.
The cycle paths of MK must be full of the ghosts of visiting cyclists, still hunting for a sign, any sign, that actually points to Bradwell, where the youth hostel is located. Those signs do exist, but no sign in MK points to anywhere more than a mile away, so when you find them you are so close that you don't really need them any more.
Other towns are just the same in my experience.
To be fair, they also signpost "Town Centre", but not of course, the way back, unless you started out from very close to Town Centre.