lastminutetravel.com seems to have the best deals, and since we bought through them, I've been keeping a close eye on feedback. They seem to be just fine, and the voucher itself (don't confuse it with your basic reciept after you buy) is sent about a week before your trip, typically. You can request it sooner, if you choose.
The posters that have already been to DL seem to have had no problem with the vouchers.