I have used DigMyPics.com. As I recall when I was reading reviews, Scancafe also had good reviews. I think the only difference is that one sends the originals to India and the other used Costa Rica. To keep costs down, almost all of these companies off-shore the actual scanning.
The results were fine (I ordered high resolution TIFFS), but these were not portfolio materials, they were family slides taken by my Dad during the 50s and 60s. They did a good job of color correcting the fading slides and there were no dust issues. But, as a say, these were family snapshots, so hard to judge the quality.
At the time I considered buying a scanner and doing it myself, but decided I could not get as good of resolution and did not want to spend the time scanning individual images for weeks on end. They do take awhile, so be aware of that.
If you aren't in any hurry, I'd send a batch to whatever vendor you choose and see how you like the results before committing to 451 slides at once. Also, since they are going overseas, it's probably safest to split the order up. (I'm superstitious).