I have a lot of iPrefer points so I turned some of them into vouchers. However I read - when you get the vouchers - that they can't be used for existing prepaid reservations. I assume that means when you show up at the front desk to checkout - unless you have an expensive reservation that you don't have to pay for in advance - you can only use the vouchers for incidentals like breakfasts and minbars rather than the actual room cost itself. Not as good as, for example, Accor, where I can use my points on the website to reduce the room rate I'm paying whether it's discounted because it's unchangeable or whether it's full price because it's short notice.
So not as helpful as I thought (leaving aside the difficulty of getting access to the website when I forgot my password and after requesting the password reset email I kept getting the message "access denied"!)