In the spirit of frank discussion, I must disagree with the dismissal of the bottomless portafilter, and the description of it as a 'mulitation' (I think that's the term used...)
I 'hacked' the portafilter for my gaggia espresso (which I think may be the ONLY espresso in the western hemisphere with a PID boiler controller ) about 5 months ago, and I think overall it has been a successful experiment.
The benefits of the bottomless portafilter are, in my opinion, focused on using it as a learning tool. Errors in distribution and tamping jump out (literally - with espresso in the eye!) and help improve techniques.
There has been argument on various boards that crema is enhanced because it does no cling to the sides of the protafilter, and comes clear out from the basket. temperature? I'm not sure that the espresso is noticeably cooler, especially if one's cups are warm...
I feel that I have learned a lot about proper distribution (I use the weiss distribution technique and the chicago chop to level) using the hacked portafilter. Combined with the PID, my espresso has become much more consistent, and better tasting in the cup.
plus it looks really neat-o when the espresso starts to pour out of the basket.