I have a stainless chiller, and I love it. My copper chiller was a home-made affair that I rescued from a garage sale, and I was always suspicious of its construction. (It had submerged threaded fittings, and was not one continuous piece of tubing.) On top of that, the hose fittings were right at the top, which meant I needed to shield the hose from the burner with foil. And if those fittings were to leak, it would be right into my cooling wort. Furthermore, I hated how the copper always came out of the boil dramatically cleaner than it went in!
The stainless one works great, and is a joy to keep clean. Theoretically it should be less efficient, but I sure haven't noticed it. The tubing is extremely thin, which I think helps make up for the inferior heat conductivity of steel.
Now the old copper one is rigged as a pre-chiller, should I need it on on of those 110° SoCal summer days.