I have purchased on Dan.com over a payment plan (just 3 months which in reality is the initial payment and then 2 further payments so actually over two months and a day). Everything went absolutely smoothly as far as I am concerned.
The system allows buyers to seal a contract for domain names they would not otherwise be able to afford at the time, and of course for the sellers it must surely increase the number of potential buyers who are able to partake in the market.