I agree with the above two posters. Not terribly easy to use and they expire.
I learned that you cannot use them more than 48 hours ahead of the flight so if the flight you want to use them on is after the "expiry" of the vouchers, that's it - they're gone, unlike the one class bookable upgrades.
There seems to be nothing about this on the website but the 48 hour before departure requirement is actually in the email they send you telling you that you've got a couple of vouchers to use up. Fortunately I have two more spare ones that I can give to family members (in my redemption group) who I've bought tickets for so 2 of the 4 will find a home before they expire!