jamiesam - it's 11% at the moment (assuming your income is below the NI ceiling).
The savings from April next year will depend upon your tax/NI status. For higher rate taxpayers - assuming you have the maximum £218 in vouchers - it could be 40% of £218/month (i.e. £87.20) per parent. If you're in the normal tax+NI band it would be 33% (i.e. £71.94). Lower tax band and/or less than the NI limit per month, and the savings are less obviously.
For us with both our employers offering the scheme, but with only one child to pay for, and dh in the 40% tax band, our saving will go up by about £100 a month compared to now. Obviously that will vary if you have more than one child using childcare, or only one employer offering it, or only one of you working, etc.
For a basic rate tax payer just the £71.94 saving next year is the same as the saving now on £654 of vouchers, so I should think most people will be better, rather than worse, off, or round about the same.
One thing to remember is that for the purposes of child tax credit calculations, childcare vouchers are NOT considered as income, but neither is a voucher spent considered to be expenditure on childcare. So if you would qualify for the childcare element of child tax credit, then having the vouchers would reduce your entitlement (at a rate of 37p in the £1) so they probably aren't worth it. If you qualify for CTC, but not the childcare element, then they could be doubly worth it, since as well as the tax saving they reduce the income CTC is calculated on, so possibly increasing your CTC (although the band to get only the minimum is quite wide so this won't work for everyone). As far as I've heard these rules aren't changing for next year, but you'd have to check with the IR to be 100% certain about that.