One of the reasons Ultimate Power is really fascinating is that it almost links to the "regular" Marvel Universe, also known as Earth-616. It doesn't quite... but almost. Reed Richards sends some probes into the Negative Zone and from there to other dimensions. Next thing we know, Straczynski's version of the Squadron Supreme pops up to fight the Fantastic Four, and The Ultimates and the X-Men get involved as well.
A little background; the Squadron Supreme originally debuted in an old Avengers comic as an alternate universe's premier superhero team. They are quite obviously an ersatz Justice League, with transparent analogs of Superman, Wonder Woman, Batman, the Flash, Green Lantern, etc. The universe they come from is Earth-712. When the Squadron Supreme was rebooted, it was the darker, MAX comics line, and the characters were redesigned and rebooted in a way not too unlike the correspondances between the Ultimate Universe and the regular Marvel Universe. So, since the Avengers have a fight with the original Squadron Supreme back in the day, it makes sense that the much darker Ultimates would have a fight with the much darker rebooted Squadron Supreme here, in a correspondance/retelling of that original story.
Where it gets even more interesting, however, is that the Scarlet Witch uses her reality altering powers to pull the original Squadron Supreme in to join the fight. These guys, not expectedly, mistake the Ultimates for the Avengers, which they had met previously, and we have... almost... a connection between the Ultimate Universe and the regular Marvel Universe.
Anyway, the Squadron Supreme stuff is what I'd like to read still, and which I've now requested from Interlibrary Loan. Again; one of the interesting things about it is that it's a dark take on the Justice League, really. Marvel created at least three different ersatz Justice Leagues that I know of--including the Squadron Supreme, the Imperial Guard, and one in the Marvel Zombies universe. There's probably more that I'm not aware of; and frankly, like I said, the Squadron Supreme comes in two completely different versions. Anyway, I'll probably talk a bit about the Squadron Supreme stuff when it arrives, because hey--I'm kinda interested in it right now.
Ultimate Power was pretty good. Bendis and Straczynski's stuff was good. The end of it, which was written by Loeb, seemed a bit confused; I'm not sure that I followed exactly what was happening all the time, or thought it was clearly written, and some of the characters seemed to suddenly pop up in the middle of nowhere with relationships that I didn't get (Kitty Pryde kinda flirting with Shape, for example.) Gary Frank, the artist, is also quite good--in fact, his style reminds me strongly of David Finch's artwork. Since Frank is the normal artist for the Squadron Supreme and Finch is the normal artist for the Ultimates, either of the two of them was a good choice that felt "at home" for either of them.