First off - love the fact these stories are collected in a compendium. You get the first 25 issues of the JSA reboot plus a crap load of extra issues in ancillary titles/stories/mini-series. I believe this covers the same contents as the JSA omnibus but at a much cheaper price. And - a lot less weight. The price point and the lighter weight is a plus for me - but the cheaper binding and fact it is a softcover might be a negative for some.
So - I had read the first 25 issues of the JSA before (6 years ago) and my feelings back then are the same as my feelings now. Johns is a very solid writer but one I don't seem to like as much as other fans do. I'll summarize his strengths: He respects the past history of the characters and the book, he is great at making the team feel like a team, he tries hard to have villains that challenge the team, he tried to find clever solutions so the heroes win out in the end, the tone is lighter and more fun than a lot of his contemporaries. There is still drama but the heroes are heroes and they win in the end. Some of the characters he creates are excellent updates to the classic characters: Mr. Terrific and Dr. Midnight were my favourites. The Hawkgirl reboot less so. And the Hourman reboot (a leftover from Grant Morrison?) made no sense at all. Weaknesses: Too many heroes and not enough character development. To be clear - he does develop some characters and when we get those moments they are excellent. But others are just touched upon and really needed more development. Some heroes were a bad fit to start with and are randomly dismissed from the team (they choose to leave, but we all know it is the writer giving up on them). Hourman and Starman, in particular, never fit well with the team and their departures are very welcome. Also, there is too wide a gap between the heroes powers at times. Now - this can work if the writer is clever but sometimes it means the hero is useless in the "big battle". For example. Mr. Terrific might not be able to summon an energy blast or run at the speed of light - but he uses his intelligence to save the day. But Black Canary (another non-powered hero) - just seems to be an observer for most of these stories, not doing anything. I contrast that with (one of my fav titles) the New Teen Titans. Yes - Robin is no where near as powerful as Wondergirl or Starfire - but the writers (Wolfman and Perez) always gave him something to do in a fight. It helped that not every villain was a super god in the New Teen Titans. Almost every other villain in the JSA seems to be able to destroy the Universe - so of course Black Canary and Wildcat and even Hawkgirl can't add much to the fight.
The biggest plus for this collection is my biggest complaint. Let me explain. While some fans will love all the extra stories and mini series included because they are completists, I found all the tangents away from the main JSA title - distracting and not particularly well written. I would rather have had those pages be filled with more JSA issues or even dropped from the collection. I know that seems weird to say "give me less!" but these extra tales just weren't that good (IMO) or out of context so I didn't get the full story (a couple of issues from Star Spangled Kid or Hourman or Young Justice) so if they weren't in this collection I would have been happier. Especially the Young Justice issues seemed so out of place. Sure the JSA showed up in the story but it was not a JSA story and you didn't need to read it to follow what was going in the ongoing JSA title (i.e. if it wasn't included - no worries).
Overall - I really enjoyed the JSA stories (JSA 1-25) and a few of the mini series were fun but I would have preferred some side stories were dropped from this imposing collection. But nobody can say you aren't getting an amazing value for your dollar if you do buy this collection.