ええ! お箸はお上手ですねぇ!
Ee! O-hashi wa o-jôzu desu nee!
Wow! You're really skilled at using chopsticks!
If I had ¥100 for every time I've heard that, I could pay someone to feed me instead of having to use chopsticks myself. But as I don't, I'll have to content myself with a commentary, a response and an anecdote.

The Commentary

The basics of using chopsticks are, indeed, very easily learned -- most people grasp the concept within a single meal, although it does take a week or two to learn to apply just the right pressure, both to grip the food securely and to stop straining your hand. When you can eat raw silken tofu with chopsticks, you'll know you've made it to the first level.

However, there are a number of food items that require some serious chopstick-fu to eat. Number one on the list for many is small fish served whole, either grilled in salt and served hot (shioyaki) or, worse yet, the half-dried kind the Japanese love to eat for breakfast; these require the ability to cut with chopsticks (this involves pressing the tips together and poking up and down like a sewing machine), pick out small bones, pry up larger ones, and keep the fish on the plate at the same time. Other perennial favorites include most things floating in oden, esp. whole boiled eggs, and hamburger patties.

Nevertheless, I've never gotten compliments for successfully tackling something difficult to eat; they tend to get rolled out when I end up with fish bits over half the table, eggs on the floor and tofu in my lap. Then again, this makes sense in a roundabout way: only when they see you being visibly inept do they remember that using chopsticks is (or should be) difficult, and thus you must be complimented for making the effort.

The Response

liontamer's suggestion isn't bad, although I'm not sure I could pronounce tsukamaeraremasu too well after a few flasks of sake -- I'd probably end up saying I can sodomize (tsukaeraremasu) flies with chopsticks instead.

But a friend of mine has developed a wordless and devastatingly effective response. When complimented, he in proper Japanese style humbly disagrees and says that he is an abject failure who couldn't chopstick to save his life.

Then he switches his chopsticks to his other hand and continues eating.

The Anecdote

So one infernally hot day in July I found myself in Tunis. I'd spent the day pottering around the ruins of Carthage with two Japanese acquaintances I'd met at the hostel (I still remember having to translate the words "votive stelae" into Japanese), so in the evening we returned to the city and went out for a bite to eat.

The meal was unremarkable to the point that I can't even remember what we ate (undoubtedly something involving harissa), but dessert was a slice of honeydew melon, a ludicrously expensive luxury in Japan with prices reaching ¥10,000 per fruit -- but which costs a few fractions of a dinar in Tunisia. So I slid my knife between the pulp and the skin to separate them and then proceeded to carve the pulp into bite-sized chunks, only to be greeted with amazed looks and cries of...

ええ! ナイフはお上手ですねぇ!
Wow! You're really skilled at using a knife!
I boggled. Was this some kind of bizarre joke? I mean, we'd just eaten the entire meal with knives and forks, and Japanese people in general (and those adventurous enough to travel solo in Arab countries in particular) seem perfectly adept at using these Western implements.

But they aren't, really. Take a close look next time you see a nihonjin grapple with a knife and fork: you'll probably notice that, instead of using the slightly serrated but dull blade in a sawing motion to cut through something, they tend to push the blade straight down, the way you'd use a heavy, sharp chef's knife -- or chopsticks. And these are the only tools most Japanese use to cut. Since most Western-style restaurants in Japan serve their food presliced, the dinner knife is not really needed for cutting and the Japanese never learn the correct technique for using it. Cutting the perfect parabola needed to disembowel a slice of melon requires practice, and (as they proceeded to demonstrate) they still needed quite a bit more.