I just got done reading the entire Miracleman series and here are some thoughts:
Moore's stuff was pretty good, I liked it but I'm still in the minority who doesn't think Moore is the best writer ever. I'm just not a fan of his classics that are deconstruction of superheroes. For me the series doesn't really pick up until the last issue of Moore's run and then Gaiman's run.
I really liked the Gaiman stuff, the issues that focuses on this new world that is ruled by gods and what the impact is. It needed more of this stuff. I especially liked the Andy Warhol issue with Gargunza. Then before the series ended, there was a great cliffhanger with the implied homosexuality of Young Miracleman. I really want to see where Gaiman goes after this. I hope the copyright issues get cleared up in Gaiman's lifetime so we can see the finale.
I haven't read Gaiman's run yet, though I've made a point to do so one day.
Personally, I don't think Moore's run hit its stride until Olympus. That's when Moore really started to radically redefine the superhero genre. And for all that it inspired the whole "superheroes taking over the world" stories, I still don't think any of them published since ever surpassed Olympus. I'll also say that Moore's version of Kid Miracleman has to be one of the most genuinely frightening villains I've seen in comics.
Hopefully, Gaiman will get his chance to continue Miracleman one day. I'm still hopeful that it'll happen... it's just a matter of when the legal infighting gets settled. Fingers crossed.
I like Moore's Miracleman a lot, though I don't think it's his best work as some others do. I do think he's the best comic writer ever, though (Best being a measurement of skill and technique, not necessarily equating to favourite).
And yeah, his Kid Miracleman is one creepy psycho dude - and probably the inspiration for Superman Prime, thinking about it, though Prime has a long way to go to be in Kid's league of evilness.