It's not fair to compare the S50 to the Rebel, and some people are complaining a lot about the Rebel underexposure.
Apparently some Rebels are in fact underexposing things compared to other Rebels, but in my case, the camera is doing the right thing to my tastes, and in fact I think the S50 overexposes by 1/3 or 2/3, so comparing both cameras, the Rebel takes darker images.
Image quality is in a completely different league in the Rebel. CMOS sensors have VERY low noise levels, so the Rebel at ISO 800 is similar to the S50 at 100 or 200 (depending of the light conditions) but more similar in average to ISO 100 noise levels. And the Rebel image is much film-like and "clean" in terms of in-camera processing. It supports much more heavy sharpening levels and corrections than a similar S50 image. Also in general it looks softer and with less saturated colors, but most of this less punchy look was changed if the Parameter 1 setting is used (the default one).
And, after all, I'm a long SLR shooter, so I feel "at home" again with the Rebel (I've sold my EOS 5 to buy the Rebel, but I don't miss it at all). It feels as a "real" camera In the "bad?" side, you can't use the LCD for image preview, only for review after the picture is taken, and there is no movie mode. Also the Rebel images are more thinked for postprocessing, despite the new Parameter 1 settings, including the so called underexposure.
The RAW files also are in a very different league. You have exposure compensation, and usually something between 0,6-1,5 EV info above the white clipping point (depending the RAW conversor used, C1Rebel seems to recover more info) so you can recover blown highlights. The AdobeRGB colorspace is also very useful, mostly in pictures with very saturated reds and yellows.
AF is superb. In situations where the S50 has nothing to do (even with the AF assist light) the Rebel focuses without problems, and in very low light situations, you can use the flash as an assist light. Using the flash is possible to focus in total darkness at close distances (less than 10 meters). And speaking of flash (the other major source of underexposure complaints), forget about redeye problems. They are nonexistent in the Rebel. My only 2 major complaints about the flash are: no 2nd courtain sync, and no Flash exposure compensation. The second one can be overcome using 10DRemote program from BreezeSys, but you need to hook the camera to the computer to change it. Not very practical, but useful if you want to set FEC to +2/3 to overcome the underexposure. The camera will remember the setting even if turned off.
So, to me, if the Rebel image is a 10, the S50 probably will be a 6. The same probably is true in AF speed and low-light operation.
And the least but not the less (in fact is one of the most), it's a interchangeable lens system!. I'm using the same lenses used in the EOS 5, and this gives me a lot of flexibility. Now I have the equivalent of a 18-300 lens between my 3 zooms (more than a 16x zoom, equivalent to 29-480 in 35mm).