"Bucket Of Hours"?

A friend who does technical writing offers his clients a "Bucket Of Hours" deal.

The client chooses a number of hours, pays for 1/3 of the hours up front and the
friend commits to doing whatever the client wants whenever they want it done
over the next 12 months. The client's obligation is to use up all the hours in
the allotted time or pay for them. The friend's obligation is to be available
and perform the request et tasks.

He says he's had success with this marketing tool.

Anybody tried it with their computer development clients?
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PeteCresswell