I am disappointed with the MoS in SBG. Considering his importance and close ties to Sauron I really would have thought he would have been more potent, and cost appropriately. He was more than just a cool looking dude that rode out of the Gate near the end of the story to get his head chopped off (in the film). He was even intended to take control of all the lands and people west of the Anduin up to the Gap of Rohan which were to be a tribute to Mordor, ruled by the Mouth from the rebuilt Isengard. There is also a feeling among some readers (myself included) that the Mouth was the rider that delivered the messages (threats) to the Dwarves of Erebor of which we hear about during the Council of Elrond (the BBC radio broadcast expands the role of the Mouth more specifically in this direction as well in a very cool way).
I like to subscribe to the theory that he is truly old (from the Second Age) considering the comment about him being so old his name is lost from any tale and even from his own memory. Consider Gollumn remembered his real name still after 500 years. As for "how" there are a few theories (well summarized over on
http://tolkiengateway.net/wiki/Mouth_of_Sauron ) but simply channeling the the Dark Lord's commands and existing so close to his presence may be enough to prolong his life. Even if you go with the shorter duration theory that still makes him as old or older than Aragorn and having a Numenorian of that age existing so close in the power and service of the Dark Lord should still justify a profile with a mix of combat, magic and special rules to rival or exceed Gothmog and rival the Nazgul in game terms. But obviously GW doesn't ask for my opinion nearly as much as they should...
When WotR came out GW did a better job beefing him up and if it wasn't for the really screwed up Cost/Power issues with the Nazgul I think the Mouth would have been a far more competitive model in that system. As it was I still used him regularly in my Dol Guldur force with great effect. He would sometimes be second to Khamul or the Necromancer and other times be my primary leader.
But now that I'm shifting back to SBG more I'm finding myself again disappointed. I may do a "use as" and replace one of GW's 'creative' Nazgul cheeze with the Mouth so I can continue to use him effectively and get the in-game benefits of one of the special Nazgul without tainting my own soul too much. :-P