I started reading Irredeemable with issue #1 and liked it, but after a few issues decided to switch to trades. I dutifully bought many volumes of the series and its companion Incorruptible, but never got around to reading them until recently when I realized "Hey, there's about 50 issues of Mark Waid stuff piled up over there that I would probably enjoy." Now, Waid has announced that both series are ending. Still, it's an impressive body of work worth exploring.
Until a recent crossover (not yet collected), the two titles were pretty much separate. Events in Irredeemable are sometimes reflected in Incorruptible, but it's a one-way connection and the books can be enjoyed separately. At this writing, there are 8 trade paperback volumes of Irredeemable and 6 of Incorruptible, plus an oversized & slipcased "Definitive Edition" of the first twelve issues of Irredeemable.
It's no secret that I'm a Mark Waid fan, and obviously his other fans will like this work too, but I honestly think it has a broader appeal like Kingdom Come did.