I've been using Mary Clark's Aquinas Reader for the last several days, doing research for a paper on Aquinas' use of Aristotle's Ethics. On the one hand, the book has been very, very useful: it's directed me to some key passages, scattered across a wide range of Aquinas' works, that I would never have found on my own: I would have restricted my readings to the Summa, and as it turns out, that would have been a big mistake. I owe some substantial "thanks" to Mary Clark for her work in assembling this wide-ranging summary of Aquinas' thought.
At the same time, I found several points where Mary Clark's work seemed troublingly sloppy. In one passage, she accidentally failed to translate a key sentence: with this sentence absent, it appeared as if Aquinas was badly misinterpreting Aristotle. I had to track down Aquinas' original Latin to figure out what was going on. In another, she mistakenly attributed a given passage to one of Thomas' works (Commentary on the Soul), when it actually came from another (Commentary on the Sentences). (Ask me how long it took to track THAT down.) And these were just in the few passages that I happened to check.
In other words, if you're willing to put up with some hard slogging (which is sort of inherent in reading Aquinas), this is a good introduction. But I'd recommend that you look elsewhere if you're trying to do an academic paper.