The Missing Link is pretty good, with a decent location and an interesting, semi self-contained story, but the question you need to ask yourself is whether or not it's worth money. In short, not really. It's still good, but for money we're approaching full-fledged expansion territory.
The Missing Link doesn't have any new abilities or items however, and to be honest it's pretty short. I completed it in 8 hours, but depending on how you clear certain areas gameplay may be lengthened (you backtrack a bit, so if guards are knocked out/dead you can just run through).
Again, I enjoyed it, but I'd recommend waiting until the price drops or goes on sale.