Goodbye Marbo my old friend
It's time to shelve you once again
Because an entry now deleted
Means you are fin'ly defeated
With the dreams of exploding supermen
And tank pens
I guess we'll buy some scions
So the new guard book is out, and yes the old 65-pt demo charge on demand, Guardsman Marbo is notably absent. Fear not, I expect we'll see a Catachan supplement or dataslate at some point that will bring him back in, or a dataslate of his own where he can just drop in with your Knight/Inquisition/Legion of the Damned army to save all their bacon.
Some other things gone, some things added, some things changed - some for the better, some for the worse, some just for no discernible reason.
I'm not going to go into a full review, but some things that stood out to me on first read-through.
Mars Attacks! With Tank Commanders for HQs, vehicle squadrons, some APC-mounted troopers you can do a pretty good armored company. With armies having moved away from tons of anti-tank firepower, they might do pretty well. Tack on some cheap upgrades to repair immobilized results and some enginseers, maybe a few psykers for divination buffs and have some fun.
The Blob! So many things with potential to stack. Giant blob, with Orders available. Good. Add in psykers with divination. Better. Add in cheap priests to make them Fearless and give them more buffs. Much better. And that some of the buffs stack...wow. Potential for giant fearless blob with 4++ rerollable invulnerable save, or with Prescienced First Rank Fire... or Prescienced ignore cover shots. Not shabby. You'll have layers of buffs, some of which may not work at any given time, but with so many some will, and on turns where everything falls into place you will be rocking.
Overall it isn't too different from the old book. A lot of things end up costing and working the same. Some big point changes, but they go both ways depending on the unit, so if you had a variety of units in your old army, it may balance out overall. If you went all air cav...well, you may need to drop a few things.
Aside from the two above, I think a variety of builds could work, and the buffs available could help with any of them. While disappointing that they didn't do more along the line of old doctrines or the new Space Marine Chapter Tactics to really differentiate armies, I think it's a pretty solid book that IG players will be able to get a lot of mileage out of.
And take the Backseat Driver (aka TART or Tactical Auto-Reliquary of Tyberius) if you have several order-issuing units.