Elite Bastards looked at the transparency anti-aliasing on the GeForce 6 series:

Although the performance hit of all of the regular anti-aliasing modes are smaller on our GeForce 7600 GT compared to its predecessor, the performance impact of both multi-sampling and super-sampling based transparency anti-aliasing are often very similar. 2x multi-sampling transparency anti-aliasing loses just 16% of performance on the GeForce 6600 GT compared to rendering with AA, with 2x super-sampling TAA taking a much heftier 40% bite out of overall performance (similar to standard 4x anti-aliasing rendering). 4x multi-sampling TAA also drops performance by around 40%, although that's nothing compared to the 75% drop seen when using 4x super-sampling TAA - A 20% higher performance hit than employing 8xS anti-aliasing on this board.

As far as playability goes on the GeForce 6600 GT, it's hard to argue that any level of anti-aliasing provides a truly playable frame rate here. However, it should be noted that this particular test really is a worst-case scenario for transparency anti-aliasing, so the feature may well be more usable on this SKU elsewhere.

From our testing, we've seen that overall both the image quality and performance metrics of transparency anti-aliasing on the GeForce 6 series is comparable to that already seen on G7x-based boards, meaning that aside from the natural performance advantage enjoyed by the newer parts, there are no additional caveats to be found for users of these older boards who want to take this additional functionality for a spin. As we've already mentioned, CounterStrike Source's Militia level isn't the best place to demonstrate playable frame rates using transparency anti-aliasing on any current hardware, simply due to the massive amount of alpha textures on show, which brings performance to its knees very easily. Pick out the vast majority of situations where alpha textures are used, and chances are you'll find far more agreeable results.