I use Kings Ferry to get to and from work every day, and will soon be reaching my 5 year anniversary. Nothing compared to some commuters, though, who have been travelling with them ever since they started.
Both for commuters and drivers it is very much a matter of choice, and you take certain things into account. Some people find they can't hack it and go to or return to the train - others find that despite the delays and general unpredictability of operation, they just can't stand being stuck against someone's left armpit.
From a commuter's point of view, it is more comfortable than the train (guaranteed seat, better climate control, etc), and more convenient. As others have commented, most of the Medway suburbs are quite some way away from the station. It is also much cheaper than the train - by about £1,000, even before adding getting to the station - and I don't drive. The Blackwall Tunnel is an absolute [insert curse of choice], but it does mean coaches can serve Canary Wharf directly - otherwise only accessible by changing.
Drivers also know what they are letting themselves in for - or do if they do their research. I have lost count of the amount of times I've heard the words "bus driver" and "I won't be going anywhere". They usually last about a fortnight. Yes, you don't have to take money and you're not driving round in circles, but it is a long day, the traffic is unpredictable and you have to do your own cleaning. It is for that reason that after a long period of merely taking from elsewhere, Kings Ferry have started training their own drivers again.
I imagine the situation is similar with other companies. Clarkes, Reliance and Chalkwell have all been in the business for years, from various areas of North Kent. Chalkwell do Sittingbourne (along with Kings Ferry) and Maidstone, Kings Ferry cover Sittingbourne, Sheerness and the Medway Towns, Clarkes cover the western Medway Towns, Snodland, Vigo and New Ash Green, whilst Reliance cover Gravesend 'proper'.
In recent years a number of other operators have taken off - Centaur started with a couple of coaches and now have about 6 from Sevenoaks and Tunbridge Wells, Brookline started with one return from Kings Hill and now have two (plus a Chalkwell service), Buzzlines have been running one journey a day from Folkestone for years, and have just got permission to run three shuttles as far as North Greenwich only.
The only real failure was Bayliss' service from Deal. They were persuaded to start by some disgruntled rail commuters but a journey of 3 hours+ on a good day meant it never really took off - in its final months it was operated by a Transit minibus.
Very much a question of "you pay your money and take your choice" - my journey regularly carries 40 people each day, and indeed the company claims to transport approximately 1,500 people across 38 coaches each day (averaging about 39).