Let me first say that the bar was set very high: McDermott's Quick and Easy Thai is a masterpiece.
These recipes are also delicious, and the ones I've tried have been easy. However, they're not nearly as quick and easy as the ones in the Thai book.
What makes them not so quick? In a word, ingredients. Most recipe has lots of them, so there's a lot of measuring/chopping going on before you cook.
Also, if you're a European American like me, you don't have these ingredients lying around, so a trip to an Asian market is a necessity, at least to stock up in the beginning.
But don't let this deter you! Make fish sauce and lemon grass part of your standard staples, and McDermott does have an excellent section on ingredients, how to find them, and which are most important.
The recipes also have notes on what to do if you can't find something: suggestions for substitutions or just leaving things out (though again, not as many as the Thai book.) These recipes are delicious and have complex flavors, so even if you leave something out it'll still be good.
So to sum up, this is a great cookbook, but if you want to enter the world of SE Asian cooking, go with McDermott's Quick and Easy Thai first.