I do my own scanning (time consuming and expensive up front, but cheap in the long run if you have a lot of slides and negs like I do). I do this not only to save money, but mostly because it gives me full control over the process. I don't think any of these services will provide you with a the best file you can get from their scanners. That would be a 48 bit TIFF of each image, so you can do your own color correction and pre-JPEG image adjustments to control contrast and other detail. You can even correct for chromatic aberrations of old lenses. TIFF files are large, and most of the actual hands-on work comes after the scan anyway.
Obviously, this all has to do with how much work you want to put into it and how important the images are to you. Mine are pretty valuable and the expense of a used Nikon scanner and stack feeder wasn't a big deal when I bought them (they cost a lot more now, as they are not being manufactured any longer).
The time to learn how to best use Vuescan scanning software and Camera Raw to get the most out of these files is a substantial investment, too, and I assume since you are looking for a service, you're not interested in doing so. If that is the case, then certainly go ahead and send you slides to one of these services. They do know a lot about color correction and other tweaks they can apply to the automated scanning process, so you'll get decent images back, just not images that could be as good as a scan can be made. They don't have the time for it, or you would not be able to afford the service. It's a compromise when you hire somebody doing it in a competitive marketplace. Lowest bidder kind of stuff. There may be some real artists out there who do quality work, but I'd be surprised if they charged less than $15 per image. A no compromise scan takes about 45 minutes for the scanner alone, per image...
To illustrate what I am talking about two links to the same slide, once scanned straight to JPEG and the other scanned and processed with modern software tools on a similar desktop scanner, just using the full image data the scanner returns to tweak it before saving.
Polaroid Sprintscan to JPEG and saved from Photoshop 6.0
http://farm3.staticflickr.com/2874/1000 ... 56de_b.jpg" onclick="window.open(this.href);return false;
Nikon Coolscan 4000ED to TIFF, then loaded in Photoshop CS6 via Camera Raw before saving
http://farm4.staticflickr.com/3834/1061 ... ffcd_o.jpg" onclick="window.open(this.href);return false;
You'll probably get something in between these two when you send images out for bulk scanning, because they usually run stuff through a Coolscan 5000ED, which is a little better in shadow detail than my 4000ED, but without processing each image before saving to JPEG, a lot of that detail will be lost before you get the files. Look at the difference in the dark areas between those two scans - that is where the biggest difference is between a good and a bad slide scan.
Perhaps you don't want to open that can of worms, because I am at a point where I will probably want to go back and scan thousands of Sierra images from the 80s and 90s one more time, because the last time around I did what these labs are doing...