I know I've been on an Eldar kick recently, but I still love my Space Marines. I picked up the new codex and gave it a good read-through. I have to say that I'm pretty darn pleased with it. I'll get to a more thorough review once I play some games (and finish the Codex: Eldar review), but here are some initial thoughts.
The Best Parts:
1) The Chapter traits
The Chapter traits are exactly what I wanted to see. They're fluffy but still very useful, and they seem pretty well balanced. The Ultramarines give them tremendous flexibility, the Iron Hands are as tough as they should be, and the White Scars are tremendously mobile. I'm particularly happy with the Imperial Fist rules since they boost damage output consistently. Nothing flashy but lots of utility. Traits for the First Founding Chapters are all they need, eliminating the abuses of the 4th Edition trait system. Best of all, you can now run a themed army without special characters!
2) Cheaper Units
There are point drops in many places that we needed them. Land Speeder Typhoons dropped to a reasonable cost. The all lascannon Predator Annihilator dropped to the point that it's worth taking (which is great because that's my favorite loadout). Devastators dropped to be on par with the other Space Marine codices. Sternguard and Vanguard Veterans both dropped to more appropriate costs, given their limited durability. Interestingly, Honor Guard now seem like a viable assault unit. I may convert some up for my Executioners.
3) Tactical Marines
Tactical squads are worth a special mention. They didn't actually get a point break, but you can now choose their upgrades with much more flexibility. Squads of five marines can take a heavy weapon. You're not required to take a Veteran Sergeant. The flexibility really makes them more interesting and not the same millstone that they used to be.
4) Generic Characters
The generic HQ choices now seem like viable alternatives to the named characters. Chapter Masters picked up an extra attack and wound, and can now use Orbital Bombardment when he moves (This last bit is incorrect. I misread the rule, and was corrected in the comments. Chapter Masters are still pretty awesome though). Librarians and Chaplains became cheaper, even if the Librarian is upgraded to mastery level 2. In addition, Chapter Relics allow you to make generic HQ pretty nasty, giving them some great assault weapons as well as very durable armor. There's even a pretty awesome special bolter.
Those are all great, but there are a few bits I'm not crazy about.
1) Special Characters and Chapter Tactics
You can now only include named characters in armies that share their chapter tactics. This makes complete sense fluffwise and cuts down on some previous abuses. However, they didn't add any new characters, so this becomes very limiting to non-Ultramarines. In the previous codex, Ultramarine characters could slot into any force without interfering with Chapter Tactics. Now, other Chapters can't take superior Librarians or Chaplains. They don't have veteran tank commanders or scout sergeants. I hope the rumored supplements do something to fix this imbalance. Also, where's the Iron Hands' special character?
2) Bolter Drill Limitations
This one may seem nitpicky since I'm using the Imperial Fist traits, but I cannot figure out why Bolter Drill doesn't work with Sternguard ammunition. It would hardly be overpowered if their accuracy improved a bit, and it makes sense since Sternguard are fluffy Fist units (especially for the Crimson Fists). I will grant that it might be too good with vengeance rounds, since you could essentially ignore Gets Hot, but it could still work with the other rounds. On a similar note, I don't see why the Raven Guard traits don't let jump infantry Scout.
Aside from those small gripes, I'm really excited to use the new codex. I'll be interspersing my experiences with the new Executioners with my thoughts on the Eldar.