I've only ever read W.i.l.d.Cats vol. 1, #1. It was clearly a case of form over substance. Lots of Jim Lee characters striking dramatic poses, and dialogs in bad need of an editor.

I hear the following volumes were much better.

I only knew about Warren Ellis's Planetary and Authority on the Wildstorm Imprint. I think his work on Authority was followed by Millar, I wonder what that one was like after he took over for Ellis.

For some reason, the first Wildstorm title I ever bought upon its release was Stormwatch #37. And I had never read an Ellis title before that, but the marketing around it got me and I'm glad it did. After that, I followed Ellis throughout his wildstorm runs, but I must confess my favorite Authority run is Mark Millar's, and by far! First of all, it had Frank Quitely on art, but he used several Ellis characters in clever ways, especially the doctor and Jenny Century, if memory serves right.
Actually, scrap that : Millar's run is only my second fave there, as Brubaker IMHO instantly became a superstar on his Auhority run, one where he showcased an ability with high concepts only mached by Alan Moore (his idea  about the 21th century Jenny, is so clever and effective that the whole run is worth getting only for that).

About Wildcats, I second the above opinion that the first Casey run is worthy, as well as the second one, which is the whole of vol. 3.
I've never read vol.4 though, don't even know what it is.

Worth noting that Vol. 1 has a fairly decent Alan Moore run with Travis Charest's art's coming of age.

Gen 13, is worth getting for the John Arcudi/Gary Frank run, but nothing else.

One of the better Wildstorm stories that I've ever read is the Captain Atom Armageddon miniseries, where Captain Atom becomes trapped in the Wildstorm Universe, searching for a way back home. The contrast between the DCU philosophy and Wildstorm philosophy was interesting and showcases all the various Wildstorm teams and characters rather well.

While I agree that Wildcats V.1 was mostly crap, I will tell you that I re-read the Alan Moore issues every few years.  Gen 13 had a light whimsical feel to the first 50 issues or so and was enjoyable. Very nice good girl artwork from Campbell, Gary Frank , Rios and Benes. Just a fun series.

I haven't read it; but, you are usually on solid ground with Alan Moore.  The man actually made a Rob Liefeld comic worth reading (helped by the fact that Liefeld wasn't doing the art), in Supreme.

Moore got to play with things without restraint and actually uses the opportunity to do more old-fashioned comic stories, with a more modern take, in his Image work.

I only read the first couple of issues of WildCATS and hated it.  That was pretty much my reaction to all of the initial Image offerings, from the founders.  They were derivative of what they had been doing at Marvel, minus the coherent writing (well, except McFarlane, who didn't even have that on Spider-Man, when he was "writing.").  The only books I stuck with were things like The Maxx and Shaman's Tears, which had real writing and something different.  I can't speak to the later volumes of WildCATS, as the early stuff was enough to make me avoid the title.  Never picked up Gen 13, as it looked like any  one of the thousand X-Men rip-offs.

Stormatch, under Ellis, was a breath of fresh air, marred by the typical (for the time) Image art.  Ellis immediately started doing more mature things with the book; but, the art wasn't always up to the level of writing.  That slowly began to change as he pushed more and more into more mature themes and more unique approaches to superheroes.  The constant costume changes didn't help, though most started with some really awful designs, with some improvement by the end.