I and my fellow philosophers have found the Clarendon Series to be quite helpful all around, especially in the case of the Categories. Instead of being adjusted to fit English idiomatic expression, Clarendon translations are quite literal (to the apparent chagrin of some). With this translation of Aristotle's Categories, one can develop an excellent picture of what Aristotle really said (without reading the Greek itself); and with the supplementary help of the extensive and thorough commentary, one can also develop a sense of where Aristotle was going (i.e. what issues he faced at the time, which concepts later philosophers borrowed and either used or abused, and plenty more). Overall, very helpful and highly recommended.
Not only is the translation second rate (Loeb or McKeon is definitely the way to go), but the commentary is awful. It does go over many of the questions but his conclusions and ideas are quite false, and easily reconizable to anyone who has given any thought to the subject. If you are interested in getting to know Aristotle, then the way to go is with Jonathan Lear's "Aristotle: The Desire to Understand".