Yeah ... these things have been around for a while. We have had one at Maling Room and, earlier, at Maltitude for a few years now. We only really use it for training and diagnosis to make sure that we arent getting chanelling: http://malachi.coffeed.com/naked-diagnosis.html
Flavour-wise, were not massive fans. The espresso comes out a bit more voluminous, which seems to make it a bit more difficult to taste the characteristics of the espresso ... sort of like a carbonated beverage. If you bang and swirl the demitasse a bit it gets it more or less back to normal. I agree with Greg Kaans theory that most taste improvements that people notice with these are really due to a failure to compensate for the added volume leading to them really pulling slightly more ristretto shots.
In a commercial setup, where you often need to split a double and where the portafilters are cleaned all the time, I dont see that theyre that useful. At home, if you base your drinks of a double, they can make it easier to keep your espresso free from oils trapped inside the portafilter, simply because there isnt a bottom to collect dreck. Also, if you are desperate to use a 3x basket and your portafilter is too small, you can use a bottomless to enable you to fit it in.