Hi Leigh;
I have taken photos of slides as a way of quickly documenting and distributing them, so I didn't spend a lot of time on the setup. As Don points out, you'll always have distortion in the geometry of the image, and Jules is right about the saturation, too. Here's an example of how mine came out:
The results could be made better by taking more time with the setup. Mounting the camera on a tripod directly behind and above the projector would remove most of the distortion, and make sure that the screen is square to them both. You'd need to experiment with the length of the exposure to get the best results, and take a manual white balance with just the projector light if you can. Also make sure that you're using a good screen, because any surface imperfections or colour cast will be captured along with the image. All of the creases in my sample were introduced by the 36" white reflector that I was using as an impromptu screen.
Here's what the same slide looks like with a scanner:
As you can see, there's a bit of a difference. Not only is the geometry perfect and the sharpness vastly improved, but it also has the correct colour and tone. Look at the highlights on the trunk of the car: in the projected slide it's blown out and featureless, but in the scanned slide it's detailed and separate highlights. The scan also is much higher resolution; scanned at 4000dpi, it makes a 48MB TIFF file that happily produced an 8x12 print. That size is pushing the limits of a 38-year-old 35mm snapshot even when printed directly, so that's not a limitation of the scanner.
I'm using a Nikon Cooscan V, which is a dedicated transparency scanner. It scans one slide at a time and it takes about five minutes to do one slide, from taking it out of the box to saving the finished file. Flatbed scanners are cheaper and can scan batches, but don't give the same resolution.
A third option may be a slide holder attachment for a macro lens, or one that can fit on your camera if it's an all-in-one with a macro mode. This lets you photograph the slide directly with your camera. I've never seen or used one myself, but it may be another option for you.
Welcome to shuttertalk! Let us know what you decide and how it turns out.