Ive done some of this work Mick, however, like you I was only dipping my toe in so to speak to see how it worked out.
One thing is that it is messy work, big jobs need 2 people IMHO, so you need to price it right depending on how dirty the driveway is, if its covered in weeds etc.
I also think you have to go out and do a lot of canvassing for this type of work to get enough work to justify buying the equipment. There are a lot of people who are going round canvassing all over the place, travelling a fair distance to get the work.
If you can get enough work it does pay good money, I got a few window jobs off the back of it as well. A good starting point is your existing customers (if they have driveway that need cleaning) you will get referrals from there, it will also give you an idea of whether you fancy it before you dive in with some expensive equipment.
Cheers
Steve