Programs: Hilton Gold; UA 3P; Cattle-Class on everything else.
Its quite easy to use the Hertz ZE 1 award certificates. Last I used them, it was necessary to get paper certificates. (I don't think this has changed, but I haven't been following it closely).
Call Hertz #1 Awards 800 number and request the appropriate certificate... They'll give you the PC # corresponding to the certificate. Use this PC# and make the reservation. (Check the black-out dates on hertz.com first - these are never really a problem for me...)
Programs: AA Gold 1MM, Marriott LT Gold, HH Diamond
To answer the other bit about "Are they hard to redeem?", as long as you stay away from the blackout dates, they are generally fairly easy to redeem. I've never had a problem using them, and I've used the 1-Free-Day certs for what would have been very expensive HLE rentals in Honolulu, San Francisco, Denver, etc.
If the points are expiring, just get the certificates issued. The expiration date on the certificates will be sometime in 2005...