First off, the book is fantastic.  BoLS has a quick overview, but I'm really impressed by the physical book.  It is huge, beautiful and well put together.  I feel like I got my money's worth in mass, if nothing else.  While waiting for the book to arrive, I re-read the first few Heresy novels and decided I wanted to do a Isstvan III Loyalist army, centering around a core of two big Sons of Horus/Luna Wolves tac squads.  Here's what I came up with for a first go, using the Sons of Horus list:

HQ
Garviel Loken
Siege Breaker, Nuncio-vox (unlocks Phosphex Shells for the Medusa)
Librarian, Mastery Level 3, Refractor Field, Artificer Armor

Elite
Contemptor Dreadnought Talon (2): Kheres Assault Cannon, Graviton Gun
Legion Destroyer Squad (5): PF/Artificer Sarge, Missile Launcher with Rad Missiles
Apothecarion Detachment (1): Augury scanner

Troops
Legion Tactical Squad (20): Nuncio-vox, Legion Vexilla
Legion Tactical Squad (15): extra CCW, PF/Artificer Sarge

Heavy
Legion Heavy Support Squad (5): 4 Lascannons, Nuncio-vox/Chainsword Sarge
Legion Medusa, Phosphex Shells

2000 points

Thursday I played a Heresy game against Bigred's Death Guard army.  He made up his list on the spot from his gigantic collection and it was similar to mine: two big Tac squads, two Heavy Support squads (Multi-meltas and Lascannons), a squad of two wound Terminators.

It turns out the 5-man Destroyer squad is pretty soft, especially when shot at by 20 dudes with Bolters or seven Lascannons.  I liked the Phosphex Shells, but I think I'd prefer to have a character that is a little more killy and I think the S10 Medusa might be better.  I ran the Librarian with Pyromancy, but ended up with Fiery Form, Inferno and Spontaneous Combustion (too many Shooting powers).  I think next time I'll try Biomancy and try to get Endurance, which would make for some fun.  Anyway, here's the next revision:

HQ
Garviel Loken
Legion Champion, Master Crafted Power Weapon, Combat Shield, Artificer Armor
Librarian, Mastery Level 3, Refractor Field, Artificer Armor


Elite
Contemptor Dreadnought Talon (2): Kheres Assault Cannon, Graviton Gun
Apothecarion Detachment (2)

Troops
Legion Tactical Squad (20): extra CCW, PF/Artificer Sarge, Legion Vexilla
Legion Tactical Squad (15): Nuncio-vox

Heavy
Legion Heavy Support Squad (5): 5 Missile Launchers
Legion Medusa (2)

Fortification
Aegis Defense Line

2000 points

Bigred is a big proponent of the Aegis, but I'm not too sure a 3+ squad of 15 with an Apothecary makes it worth-while.  I do want a more mobile element, and if I could guarantee Gate of Infinity I'd go for that.  The next thing to add with be something that provides some mobility, maybe a Dreadclaw for one of the Contemptors and a Justaerin squad to drop in, too.

I think the Legion lists don't really come into their own until 2500 points.  If you are playing with a Primarch I think 3000 might be a good number.  Of course, I'm sure this is no accident on Forgeworld's part. 

More, including some painting, coming soon.