Oh, I forgot, don't buy the cheapest POS TC that you can find. If you're adding it to a high quality lens, then use a high quality TC. (Kinko makes some good stuff, but not all Kindos are the same high quality. All the Canons are high quality).
BTW, my EF 1.4X TC-II is for sale for $150.
(The TC-III that replaced it costs $429!!).