Caligula - Ok you want maths, you've got maths...
£50 of vouchers saves you 22% tax and 11% NI for most people (depending upon the highest rate you pay tax at, whether you're contracted out of SERPS, and whether you're over the NI threshold). If you're getting more than the basic £545 a year in credits then it's a fair bet that that's your tax situation, so the saving is £50 x 33% per week - i.e. £16.77
BUT.... If your childcare costs were already within the maximum then the amount of potential credits you'd be eligable for would go down by 70% of £50/week - i.e. £35.
However the income the credits are assessed against would go down by £50/week - which would actually increase your CTC entitlement by 37% (the withdrawal rate) of £50 = £18.50
So overall not a lot in it - better off by about 27p a week I make it - which isn't really any gain if the vouchers are part of a salary sacrifice and hence affect company pension contributions or SMP entitlement.
Basically if you are only paying the 10% tax band, pay a reduced rate of NI, or have an income of less than about £141 a week you'll be worse off having vouchers. If you're Mr/Mrs average on the basic rates of tax and NI, but receive over £545 a year in CTC there's no difference. Anyone else, you gain.
However the income level at which you go down to the basic £545 CTC isn't that high in terms of 2 earner families (don't have up to date figures but IIRC it was about £42k with 2 kids and max childcare last year) so there will be a lot of people who fit into that "gain" category.