I've bought two Seat Unique tickets to the United game in February. I'm living in Germany, so I'll only get to the odd game. When you add the membership price, it's actually not all that more expensive than a normal ticket. But, as you say, it gives you a guarantee of a seat and you can make some of that extra money back by being able to plan hotels and flights further in advance.
I'm sorry, but I am not entering a ballot and hanging around hoping to get a late ticket when I want to go to one match a year. Leisure should be relaxing - not stress. I had the membership last year and still couldn't get a ticket to Goodison, so that was that for me. I'll pay the extra few quid for certainty.