Ok, I just ordered the rules compendium and pre-ordred the monster vault from amazon. So, new question...
In terms of session planning and table book needs, will the compendium replace the original phb and dmg books rules and mechanics wise? I assume the original books are still needed for classes and flavor, just altering in favor of errata but for live use the essentials books are what I need?
Also, how does the monster vault work in light of the changes in mm3? Does it replace the first 3 books in favor of essentials monsters or is it just a parallel monster list?
The RC definitely replaces the PHB1 entirely as far as general rules go. It doesn't provide much in the way of encounter building guidelines and doesn't cover things like building monsters or NPCs at all. It pretty much lacks rules for things like stunts as well, though it does a good job of generally covering skill use, 'page 42' doesn't have an exact equivalent. For use at the table it pretty well does the job, but you really would want to have either the DMGs or maybe the DM's Kit for additional material related to designing adventures. I haven't got a DMK but even there my impression is it is a lot less oriented towards building your own encounters and adventures are more towards stuff you need to know to RUN them. For an experienced DM I'd say these books are fine. Ironically for one just starting out I'd still have to recommend the 4e DMGs as much more thorough and containing some really good info Essentials skips.
Note that the RC has general rules for pretty much everything, but it lacks ANY kind of item content. They cover magic items, but there are none in the book. They cover weapons and armor, but there are no weapon or armor tables. They at least mention rituals, but there are none in the book, same with traps, terrain, etc. It is strictly a 'what is the rule for this' book.
Looks like the MV is a perfectly fine stand-alone monster book. It overlaps with the MM series to a limited extent, for instance there are a bunch of new versions of dragons. In a lot of cases it presents new variations of classic monsters, like humanoids, which you can use as your stock orcs and such, but the older versions are still around and could be used together with the new ones. The new monsters conform to the newest monster design guidelines and are more polished than (particularly MM1) monsters. Still, a lot of old monsters are perfectly good. I don't see a TON of reason to get MM1, but you may find some gaps in your lineup without it. If you have DDI/MB then you have those statblocks though. MM3 is a good book with pretty much modern fun monsters, you might consider it as a good supplement to MV. MM2 is a bit more in-between. The monsters ARE more refined than MM1 monsters, but they aren't as slick as the new monsters and don't have all the updated defense and damage tweaks. Again, if you have an MB then MM2 probably isn't that needed.
Obviously if you have RC, DMK, MV, and an HotF* book you're pretty much fully covered. I still think the DMGs are worthwhile, and MM3 is probably worthwhile, but the other books probably not unless you desire to play older classes. Sounds like PHB1 classes will be reprinted in Feb, so that book is probably just not worth having at all anymore. PHB2 has a lot of good stuff. It may or may not get rehashed but it could be a while. That one might be good to grab if you can. PHB3 meh. I mean if you want psionics then that is probably the only place you're going to get them in the foreseeable future. The * Power books could be fun if you have PHB1/2 etc. Certainly not vital if you mainly play Essentials.
(I was thinking of one DM I've played with who was spending his entire encounter budget on one or two higher level monsters - often soldiers. 15s to hit were not uncommon - one monster we faced at third level had AC24 - and another was a roc that gained the ability to hover...)
