Phoenix,
To be honest, the primary influence on a child's work ethic, character, aspirations and attitude to school are you, the parents.
I speak as someone whose siblings (for reasons I won't go into here) went through educational paths from very academic private school, state ex-secondary modern followed by private 6th form, to state (just) ex-secondary modern [last SM intake still in the school when sibling started] followed by state 6th form college ...and all went to Oxbridge to get very good deegrees including further degrees.
Why? Because our parents went to Oxbridge, and expected it of us - and to a certain extent, knew how to get there enough to ensure that none of the barriers indifferent schooling may have put in our way were insurmountable.
Whether pre-prep is worth it depends on a) the pre-prep in question (do not, as a previous poster has so eloquently pointed out, assume that academic standards are higher just because a school charges fees), b) the state alternatives and c) your ability and willingness to do some of the educational legwork (e.g. reading to / with your child every night) yourself.
Pre-preps, in my experience, are more formal than the same years at a state school. Reception, to a parent expecting school to mean rows of desks and an omnipotent teacher at the front, looks particularly alien - despite being an absolutely fantastic educational environment. if you knwo that you want an education for your children that 'looks like your own', then a pre-prep is more likely to provide it. It may not be better than the education provided in the state school next door, but it will be more amiliear to you. If you are willing to be a little more open-minded, and if your local state schools are good, then those may be a more lively educational environment.
it also depends on the selectivity of the next school - for highly-selective prep schools, a pre-prep that offers several years of coaching for the entrance tests may be a good investment (though it is interesting that an acquaintance of mine is FUMING because several state school pupils have got in to their chosen prep school and her child - from a private school - has not. 'I paid money, my child should get the place').