I've seen several negative reviews on it, with some fairly unreasonable criticisms (the outfits looked dumb, the JLA isn't "supposed" to be a covert team, it sucked compared to Heroes, blah blah blah).
On the other hand, I loved every minute of it. Heroes has set a new bar, but I think this episode was solid and fun. I don't know what people expect on a TV budget, frankly, but I thought it was an excellent episode. Decent (though simple) plot, Lex being his full-on bad self, good interaction between the heroes, and a satisfying conclusion. Not sure what else you could want.
I liked it a lot. If handled right, Green Arrow can be so cool. My only problem was that Superman didn't join with the Justice League. I loved Aquaman's fish joke line. You're right, I wish they had Martian Manhunter on there. You know the only problem with that show? If a young Batman, about 17, was put in the same position, he could do it so much easier. In the JLA one, Superman breaks in, is rescued, helps Flash/Impulse, sets off a bomb, whatever. Bruce Wayne at that age could do all that so much easier and he wouldn't have needed everyone else to help him. It was a good decision to put Cyborg on the show, too-I wouldn't have thought it would fit.