I've just used 2 of the "old" premium upgrade vouchers which were well worth it IMO.

Wanted to go to Orlando at fairly short notice, 10 weeks advance or so. Economy flights were £790 return from Glasgow, PE was £1350. Managed to find 2 award seats in PE (you only need award seats available in PE, not in Economy), PE taxes were £380 each plus 25k miles for the economy redemption. Really good value, effectively made the voucher worth £720 in my situation (if you value the points at 1p).

YMMV depending on when and where you use them I guess.