Spare parts really only runs on android v1.5 and 1.6 which is prob why many of you haven't noticed any real advance in performance.. eris and droid run on 2.0 and nexus1 on 2.1 which, hopefully are fast enough where it doesn't need it.. tho this forum sounds like that's not the case unfortunately.. but I'm sure the dev will come out with an update for y'all.
And GDE is a customizable home/desktop replacement app similar to pandahome, freshface, home++, etc.. They all have their perks with different special widgets and options that are unique to the app. You can download different themes for your home with skins like nexus1 or hero among tonss of other themes and they come w wallpapers too. GDE let's u have up to six different desktops with new custom transitions like cube and stretch and custom widgets like an application dock and phone history. Some of the other apps give u the option to change ur icons depending on the theme but I'm not really into that. Overall tho, GDE seems to run the cleanest and smoothest for me, especially running with Spare Parts! Everyone seems to have their own opinion obviously.. it really just depends on what u wanna do so try em all!
And they all are going to work ur battery a little more than ur default home, but it was nothing detrimental...