My experience is more of hitchhikers I've carried, than of hitching, myself: certainly not done it since my student days. I can recall one effort to get from Leeds to London, when the bus was full up and I couldn't afford the train. This was way back in about 1970-71.
After what seemed like hours standing forlorn at the roundabout, fairly pleasant enough guy in a Ford Anglia (think: flying car in Harry Potter) stops for me. As we get going on the motorway it seems to me he's pushing it far more than you'd expect an angle-box to be capable of, also car's very noisy with a strong smell of petrol mixed with exhaust. After asking him about this, he admits to having prised in a 2-litre engine out of a totally different car, no mention of whether he did anything to the transmission or suspension (forerunner of Barryboys maybe?) ... anyway we're belting down towards Sheffield when there's a big 'clunk' and looking back I see a small pulley spinning off to the side of the road. Turns out the fan/alternator drive has come adrift, immobilising the car and he has to call for a tow. I have no choice but to stick with him, it being middle-of-nowhere on the motorway.
He gets the tow to some small back-street garage, they (after a lot of sweat) manage to re-fit the pulley but warn him that the big-ends are about to crack up, at first they refuse to let him drive the car off but after a lot of argument they demur. I at this point want to leg it to the nearest bus-stop but he's very insistent I stay with him (why?). So back onto the motorway: despite warnings (and my protestations) he's up to his old speed-merchant habit again, the noise getting more and more unendurable.
Eventually something's got to give and it does! Somewhere near Chesterfield, a piston-rod comes loose and stabs a hole right through the cylinder block, dumping oil, water, the works in a cloud of smoke and seizing the engine. Another call to the breakdown truck. At this point, since I can see the junction less than a mile ahead, I insist on parting company and walking up the slip road to the roundabout, where I stand and wait - and wait (it's getting late in the day now).
Lo and behold, another car stops for me. I glance at the driver and think 'you look familiar'. Yes - he's had the car towed to another garage, abandoned it, and hired a car to complete his journey - and stopped to pick me up a second time.
So we reach Coventry - his destination - without further mishap, and I take the train the rest of the way (costing me more than the bus from Leeds would have). I was out of pocket on this journey (I offered him a bit for the petrol, I felt I had to: but he refused). Goodness only knows how much this wretched driver was out of pocket. But I never heard from him again...