gameSpace - The Quick Low-Down...
(3DS Max fanatics - nothing to see here... move along now)
There have been a few issues with gameSpace v1.0 since it's release: slow startup times (1 min 20 sec for me on one system). This is due to some issue with the stock plugins, and has been resolved already for me and some others who were bold enough to use the "Win98 Patch" on XP as well.

Another issue: the .X exporter randomly hangs gS. This is unfortunate for DBP users like me.
There will be a service pack released this week, and hopefully it will address the above issues and more.
In general, gS is a *serious* modelling program (trueSpace has always had more bang for the buck than any other modeller, IMO). Caligari has gotten their ".X format" act together. gS can export IK animation, bone+skin animation, and will copy texture files to the output directory. Three key features that most other apps don't do (at least altogether). (3DS Max fanatics - go away! I will ignore you!). tS has had sucky support for .X in the past, but with gS, they finally got it right.
Features...
gS has a SLEW of features... just check out the site, it's pretty much accurate (if a little vague). As someone mentioned, it's based on tS 6.6, so it's got the full monty in there for modelling - not some old version prettied up a bit. However, if you want to do ray-tracing, radiosity, AVIs, etc., gS is NOT what you want - the rendering features from tS 6.6 have been taken out. If you want them - go get tS 6.6, then wait for "gamePack" to be released to give your tS 6.6 the gS format import/export capabilities (for significant $$$ I might add).
For the beginner...
Well, I'm only beginning to scratch the surface of gS (mostly just lack of time so far). The interface, while some don't like tS interfaces in general, it has been changed, enhanced and fine tuned over the years, and it is now more sensible than ever with gS. Yep... there are tons of buttons with little pictures that you don't know what they mean at first, but you get used to them. And if you don't like the way things are laid out... you can change it all to suit you.
The object manipulation tools make more sense than any other app I've used... wanna stretch an object in a direction - you just go for the "cage" on the end of the object that makes sense, grab it in the right place, and voila, your stretching. You don't have to move the mouse all the way across the screen to click a button to change to "stretch" mode (or remember which F-keys to hit), and then click two other buttons to specify which axes to allow/disallow - you just click near the object on the "cage", with a particular button, and move in a particular direction to get a particular operation. At first it was all very daunting, but after going through the first few tutorials, it all becomes clear that this stuff makes *sense* and minimizes the time wasted fighting with the interface (Milkshape, 3DC, etc., all suffer from rigidity of the interface - you'll understand when you start getting the hang of gS).
Game formats...
Given there's a special Milkshape and "fire off MS" plugin there in gS, you get all the MS formats for import/export (and that list is extensive). Sure, it'd be nice to have gS itself support them all natively, and Caligari seems to be motivated to add these natively to gS, but I personally don't want to wait to use gS while they implement them all. Plus, Caligari, as wells as the current gS user base, would prefer Caligari spend time on support for *new* game formats for now. If having MS embedded in there is too "ugly" for you... you might want to look elsewhere. I don't need those formats, so I don't even notice it's there. One point of note though... some have claimed that there is a poly-limit in the special MS that could give you trouble, but IMO, it sounded like it was plenty for "low-poly" modelling and animation. School up on the MS issues yourself if it's a major buying point.
Boned animation...
Everyone wants it. I must say, gS has some pretty slick features for boned animation. In my brief tests, it's pretty darn easy to add bones to a mesh and start animating it, complete with skin deformation. Unfortunately, at some point (probably patch 5), the DBP .X loader got broken and patch 5.1b can't load a boned animation (boy was I ticked after making my first boned animation with gS - and then trying to load in into DBP!). But Mike has assured me on the bug reports forum that the new .X converter DLL will be released soon and it can load boned animations.
Textures...
gS does have some very nice procedural texturing capabilities, and some really cool ray-tracing-style "shaders" for creating materials (not to be confused with DX8/9 hardware pixel and vertex shaders - no support for that in gS). The bad part - this is no good for use on 3D objects that you want to export and use in DBP (or most any game engine). The good part is that you can use these shaders and procedural textures as a "texture studio" to generate your own texture maps... complete with bump-mapping, transparency, and all the standard stuff that looks really cool in a ray-tracer. A feature called "texture baking" can be used to apply these pre-rendered texture maps to your objects, then your back in business. I haven't gotten that far with gS yet however, so I don't know the details of how it all works... yet...
The SDK and Python...
Well, I love Python, so that was a selling point that tipped me "over the edge" - just on general principle. Unfortunately, getting information on the actual particulars of what you can and cannot do with gS Python has been, let's say, "irritating". Fortunately, I've gotten some answers here and there and it seems Caligari is in fact going to beef up the Python support, and they've provided me with enough info. to play around with gS Python, make objects, etc. And it appears you can even create GUI elements (using the normal PythonWin module for that type of stuff) to make entry dialogs and such. It's clear from the scant docs, the Python support was written for tS 4.X and hasn't changed much at all since then. Hopefully, that will change in the near future. One final note - it's Python 1.5.2, which is pretty "old". Python 2.4 is currently in development. Still, Python 1.5.2 is still a "good" version IMO, although it'd be nice for gS Python to be kept current with the entire "world" that is Python so we can be sure to have access to all the Python extension packages out there, etc.
The "tsx" SDK... well it's not even released yet. And won't be for another several weeks. On one hand, since they claimed there was an API, as a "feature", it should've been there at release. On the other hands, the SDK is never fully ironed out until right at release, so noone wants to hold up release just for them to right the docs, and finalize the SDK. The somewhat "irritating" aspect is that in my endeavors get some real, "meaty" information about Python, I kept getting the same ole drone over and over about how "you should do that kind of thing with the SDK, Python probably won't cut it", but they couldn't really tell me specifics about why.
Price...
Well, $299 is a big chunk o'change (pre-order was only $199 - but too late for that now). From what I hear, Caligari has a 30-day money-back guarantee, so, if you don't like it, get your money back (double-check for yourself tho' before you drop the $$$). (3DS Max fanatics - go away with your "free" educational versions!!! I'm still ignoring you!!!)
Hope this helps...
Desktop: AMD900/256MB/Radeon 8500 64MB/Win98SE/DX9.0b/Catalyst 3.7
Laptop: P3-850/384MB/GeForce2 Go 16MB/WinXP Pro/DX9.0b/Detonator 45.23