In my experience, it's not really a 'free' companion ticket. I've used twice I think, ended up saving about $100 total each time. Won't always have the flights you want, but sometimes will. If you can get a ticket for $300, you can call and probably will have to pay $500 for that same ticket, but will also get the free companion ticket.
YMMV, but I think it's a perk that you'll probably be able to value around the cost of the annual fee (maybe less now that it has been raised to $125). I think now they are also offering unlimited tickets purchased through them at $50 off what you could get anywhere else.
Also, you won't accumulate any flight points for tickets purchased through their travel agency in my experience.
Hope this helps.