This was an early question of mine too.
As I've had my 300U for almost a month now and am approaching 200 hours use

, I'll chime in. I use the 300U for normal satellite (DirecTV) viewing and for DVDs. I haven't moved up to HDTV yet.
The S designation means that the image is not stretched to fill the screen. As an example, if the incoming signal is 480i (from say, a DirecTV), in 4:3 mode the image fills the screen vertically (with side bars) using approximately (if not exactly) 540 lines of resolution; i.e the image is stretched (actually, scaled).
Using the S4:3 aspect with the same signal (480i), the picture is displayed using 480 lines of resolution with 30 lines of black bar above and below the picture to fill the 540 lines; i.e the picture is not stretched (scaled), so it seems smaller in comparison.
The image is scaled or not scaled (S) in the horizontal direction too.
Same thing with S16:9 except it's widescreen, of course.
As for JUST and ZOOM, both scale the image to fill the screen (the entire 16:9 screen). These 2 aspects will affect a 16:9 signal also, but is NOT recommended as they are meant to adjust a 4:3 signal to fill a 16:9 screen.
As you've probably already seen, a 4:3 signal displayed using the 16:9 aspect has a horizontally stretched image (round objects look like ovals). If you use the JUST aspect, the image is stretched horizontally, but not in the same manner. The middle of the picture is stretched less than the edges. This makes a 4:3 signal supposedly easier to watch in widescreen since usually the important part of the image is directed toward the center of the image.
The ZOOM aspect takes the 4:3 signal and stretches it horizontally (like the 16:9 aspect, NOT like JUST), and it also stretches it vertically too. It basically stretches the image to be a 4:3 image as wide as a 16:9 image, however, since the LCD panels are 16:9, the top and bottom of the image are cut off. Note that if you watch a 4:3 signal that is in Letterbox, when you use ZOOM aspect, the parts that are cut off are the black bars of the letterboxing (which makes sense, since a 4:3 letterboxed image is close to an actual 16:9 image). Letterbox ratios vary though (1.78, 1.85, 2.35), so the amount of the black bars that is cut off using ZOOM varies from no black bars showing (1.78) to still quite a bit of black bar showing (2.35).
S1/S2 aspect will adjust the picture to the correct aspect ratio (4:3 or 16:9) automatically when it receives the proper signal from a source that can send an S1/S2 signal. I've never actually seen this work since my DVD player (Panasonic XP30) and DirecTV doesn't do this.
Of note: all aspects except S4:3 and S16:9 will scale the image to fill the LCD panels (960X540).
For my fiance and myself, we watch a lot of 480i (DirecTV). We switch aspect ratios alot as some programs are better in 4:3 mode than in JUST mode (but those are the 2 we use for normal 4:3 images). For programs that are in Letterbox, we always use ZOOM. We don't use the S4:3 mode as the projector does a pretty good job at scaling the image (the image quality of a 480i signal isn't significantly better unscaled vs. scaled, just smaller

). Hence, we never use the S16:9 aspect either.
Note that the DirecTV is connected using S-Video, so I cannot comment on a composite or RF signal.
The progressive DVD player is connected directly to the pj using 30ft component video cables. The pj menu lists the input signal for component as 525p when I play a DVD. Some DVDs require the use of ZOOM aspect to get rid of the letterboxing, and some DVDs are enhanced for widescreen and display beautifully in 16:9 aspect.
I used the 16:9 aspect to determine maximum screen size.
Hope all this is helpful in some way,
Ken