Well, there's already a review up from a Warhammer 40k Vet - so here's one from a 40k newbie:
As previous reviews have mentioned, the Battle for Macragge kit comes with a mostly-full rulebook (the only things missing from this one are large sections of back-story, a hobby tips section, and kill-team rules (a variant of "traditional" Warhammer 40k)), some great terrain pieces, dice, blast templates, and, of course, several minis for both the Space Marines and Tyranids and a scenario book for their use.
Strictly speaking, the box contains all that you need to play the included scenarios, except for superglue or model cement. If you're completely new to miniature gaming, it may come as a surprise that you'll need to assemble (and, preferably, paint!) the miniatures before playing the game. Well diagrammed instructions for both tbe building and paiting of the models are included as part of the scenario book. If, like me, you're new (or coming back after a LONG hiatus) to model-building, be prepared to spend almost as much on modeling supplies as you did on the game itself (I spent about $45 US on the game, and at least as much on glue, primer, paint, basing sand, brushes, hobby knives, and other supplies).
The included scenarios provide a fun, easy introduction to the rules of Warhammer 40k. They start off slow and simple, and each mission builds on those previous. Once you complete those scenarios, the Games Workshop website has the rules for an additional 10 scenarios, though you'll need to purchase additional models to complete most of these.
Which brings up the one apparent drawback of this kit: while it does indeed supply a couple of small forces to get started playing, neither of those forces meets the minimum requirements for a regulation game. For that, you'll need to pick up the Codex, or army book, for either of these armies (or any of the other numerous Warhammer 40k armies) and several additional models.
While at first glance this appears to be a large drawback, it's also one of the strong points of this, or any other miniatures-based game. You're free, within the confines of the rules, to build your army as you see fit. You'll very rarely see two players field the exact same army list - and you'll see players with similar army lists display very different strategies.
All in all, if you enjoy basic model building, basic (and not-so-basic) model painting, and table-top wargaming in an over-the-top science fiction setting, Warhammer 40k, the Battle for Macragge might be just the thing.