It depends, if you are shopping for a certain level horse with scores, then I would look at the overall show size, and judge and then look at the scores.
If you are looking at a young/green horse, then I think it's a good show experience as they are at the show like everyone else.
If the opportunity classes are at a recognized show under a judge you would normally be happy to look at scores from, then there shouldn't be any difference. Same show, same judge, same ring...just none of those pesky USDF/USEF fees
Opportuity classes are held at rated shows with licensed judges - the only difference between them and the "regular" classes is that the owner didn't have to be a member of anything. I would look at them exactly the same as if I was looking at the "regular" classes.