Okay, well I've finished the campaign after a good bit of backtracking midway in a futile attempt at regaining a bugged achievement or two.
My take is this:
RAGE is a flawed game made up of brilliant pieces, stitched together poorly. If you're a fan of first person shooters you should play the game, but definitely not at full price.
The Good:
For all the heat the game has taken about pop-in textures, I must say that the issue has been satisfactorily resolved for the PC version, provided you're running a reasonably powerful system. It's true that up close, some textures just never reach a convincing level of detail (Carmack has promised a higher level of texture info in a patch), but viewed as a whole, the non-repeating nature of the details of the game world is simply stunning. It does things you've never seen before and it will make it hard to "unsee" in other games.
The game mechanics are typically smooth id. Everything feels right, particularly the shooting aspects of the game. The animation is absolutely top notch and the way the enemies use the environment to reach you is really something to see. At times the AI is disguised by effectively clever scripting and an excellent mix of stand-and-shoot enemies and rushers/grenade tossers. Many of the levels are awe-inspiring in their design and some of the "boss fights" make those levels truly special. A handful of side missions are brief and entertaining diversions. The voice acting is generally very good and it was a nice surprise to hear John Goodman at the beginning of the game.
The Bad:
The enemy AI can be woefully predictable at times, often falling into an unshakable pattern of peek-a-boo behind cover. Too many of the levels offer gameplay that feels recycled and while you'll still probably enjoy shooting in them, the experience becomes somewhat lifeless. The lack of destructible/movable items in the game is glaring. The Wasteland is simply an area for car combat in between corridor shooting and other than a few trick jumps there's very little reason to explore - nor do you feel as though you may have missed something once you've driven to each indoor area. There are also a very limited number of optional missions so you won't have missed much if you stick strictly to the main path.
The menu and UI is clearly very gamepad friendly and it can seem counter-intuitive at times. The decision to allow you to carry every weapon, but limiting the rotation to four, is baffling. You're forced to fiddle with the menu every time you want to change your loadout.
Driving is a bit touchy and probably lends itself better to a gamepad. The Rally games are infuriating, though thankfully few in number. The racing portion feels very tacked-on and could be skipped almost entirely.
Don't go into this expecting an amazing story - it's not there. It's tolerable and serviceable, but that's about it. The ending is as abrupt and dull as advertised. Not every game needs a big final showdown, but you should feel as though you've overcome one major final hurdle. It's very anti-climactic.
Summary:
All in all, there are some fine - even breathtaking - moments. Unfortunately, they are punctuated by disconnected periods of blah. Shooter fans should not miss it, but wait for a big price drop and the flaws will become easier to swallow.