I've not used a service. In the past, many years ago, i've used copiers and scanners to do the scanning. It depends on how picky you are. More picky, more expensive for scanner or service.
If money and time are not a problem, you can purchase some pretty expensive slide scanners, and take your time with scanning the slides. Some scanners have automatic feed, which is nice if you are doing a bunch of them. Then do post processing of your images. I had some slides that were almost completely black looking, way under exposed and was able to manipulate the image to get an adequate picture from it.
If i was to do slides that are mostly people oriented, the quality of the image is not as important as being able to recognize the people and have a pleasant viewing experience, i'd be tempted to try a service out. The down side of sending to the services is loss of slides, either from shipping or the facility that will do the processing.
I've also used a digital SLR to take picts of 35mm slides. A stand for this is desirable. The pro for this is if you have the camera, doesn't need to be a SLR, you can try it out and see how it goes. A light source like a light table or something similar will make it easier.