OK, 50% is pretty high and 80% is very high.

Now, if your data isn't terribly large, you might barely notice any query degradation (at least to the naked eye)

But bottom line, when you reach fragmentation that high, SQL Server has to do more work than necessary to retrieve data.

I have only ever seen this once, and it was 10 years ago - but I saw a site where fragmentation was so high (approaching 99%) that the SQL Server engine basically took the database offline (or something to that effect). It was a real cluster-****