I've got an '05 GX, a '00 LX, and an '05 LX. My GX drives like a rocket ship compared to either of the LX's. Similarities are really just the motors and trans (years specified above in another post) and then other this and that items since they're both Lexus/Toyota. Really the GX *is* the 4th Gen 4Runner...same exact chassis, drivetrain (the V8 obviously cause the GX470 didn't get a V6 option in the USA), and suspension (other than the AHC air bags in the rear of the GX). (also very similar to the 07-09? FJ Cruiser...which are all the LC Prado 120 chassis...)
I would say the main drawbacks have been posted above as well: GX is quite a bit smaller than the LX (really can be seen as a positive over the LX in some aspects) and the stupid side opening single rear door. I hate that damn door, but it def would not change my mind about getting one or not. Just wish they would've done a slide down rear window option on it like the 4runner (which they did on the GX460 apparently...)
Another drawback currently is there is a LOT less after market support for the GX470 vs the LX470/100-series. Most everything ARB makes for the Prado 120 is available in USA/Canada now and a bunch of 4th Gen 4runner and early FJC stuff can fit up and there's also a *small* handful of fab shops getting behind the GX and its getting better as more and more folks start getting into the GXs as an off-roader and really, if you try hard enough, most of the stuff from the Prado 120 can fit up, you just have to get it from another country besides USA or Canada.
I mainly drive the GX and the wife drives the kids to school in the '05 LX and then we use that LX for the whole family (2 adults, 4 kids) to fit in better. Every time I drive either of the LXs after having not driven it for a couple weeks or so, it just feels like I'm driving a giant tank. A *real* Cruiser. Instead of the Land Cruiser's little brother. And I start to think, man, maybe I should get rid of the GX and switch DD to the other LX....And then I get frustrated how slow the LXs feel vs. my GX. haha. But seriously, if you want that O.G. Cruiser tank kinda feeling, you will not find it in the GX. I love them both and love having both, but they are quite a bit different.
-Ferg