Two years ago I compared DMP against another large scanning company, sending a test batch of slides to each company, swapping the test orders and sending them out again so each company was tested twice using exactly the same slides. DMP had consistently better scans, fewer or none scan line artifacts, better tones and detail in deep shadow and highlights, truer color correction and saturation, and slightly better detail in the jpegs. I also compared against a Nikon 5000, an OpticFilm 820, a $100 USB slide digitizer, and re-photographing the slides with a DSLR and macro lens.
DMP came out as the best choice for high quality, affordable cost, and time on my part of the project. During two years we have sent 41 orders in about 20 shipments to DMP, totalling almost 5,000 slides, some of the slides up to 60 years old. None of them had to be rescanned. I'm happy with all the scans.
You can buy your own slide scanner or digitizer, but if there are many slides to scan you also need to consider the time to check each one for dust and blow it clean, set the color conversion for normal or Kodachrome, rotate or flip over each slide, color-correct old or faded slides, and reposition the scanning rectangle whenever the slide isn't in the same place in the frame.
A couple of tips - read the info on the DMP site about scanning and choose the resolution you need, or ask them. Package your slides well for shipment. Use a small box so they stay stacked together and repackage into a larger box for the entire order. If they're ancient or valuable you could scan them on a cheaper scanner yourself as insurance against the slides being lost or damaged in shipping, although no problems occurred in those 20 shipments to DMP from Canada.