They are really useful, my stuff is littered with them! They are great for creating quick interiors, ships or scenes when you haven't got the time or the inclination to model up from scratch (cheating really

. And good fun just to play with too.
The ships in these are made up of Powerfusion parts:
http://web.me.com/dredington/Traveller/gallery/ships/the_devils_trill_3.html
http://web.me.com/dredington/Traveller/gallery/scenes_of_the_third_imperiu/escape.html
And most of the consoles, screens and seating in these:
http://web.me.com/dredington/Akhilleus/ship_interiors/the_bridge_of_the_akhilleus_2.html
http://web.me.com/dredington/Akhilleus/ship_interiors/the_bridge_of_the_akhilleus.html
Their vehicle parts are good too:
http://web.me.com/dredington/2300ad/2300ad/henderson_explorer_mk_iv.html
I'd recommend you get the items which contain lots of parts and you can get to be a bit creative with your designs.
I always had good service from PF3d and SGS, whenever I have had download problems or lost passwords (a frequent problem) they have sorted it out straight away.