There are a few options ranging in price and quality from cheap/low to $$$/excellent.
First is to project them and photograph them, quality can be better than we would expect and cost is low (if you have a projector). Speed is fast.
Next is a dedicated slide copier lens that attaches to a dSLR. You really need a full-frame dSLR to get the entire slide in the photo. If you have that the cost is maybe $50 for the lens, quality is good, speed is medium.
Next is a flatbed scanner such as Epson V500/V600. Cost is about $150 to $250, quality is good, speed is slow.
Finally we have a dedicated slide/negative scanner. Cost is high and quality is great, speed is slow.
I have tried all of these and for now have settled on the copier lens as the fastest with good quality (sufficient for full screen on a monitor).