This is a bug report. I already worked around it on my end. But it seems pretty serious so I figured I should report it.
1. Create a custom post type with a custom single line input field called "region"
2. Make some records that include a region with an ampersand, e.g. 'USA & Europe'
3. Create a parametric search which has a dropdown filter on "region" and which updates via ajax
Result: If you select 'USA & Europe' you will get 0 results.
For example, look at this screenshot: hidden link
Originally I was using the region name "Northern CA: No. & E Counties" and I would get zero results. I re-imported my data and used the label "Northern CA: No. and E counties" and now I get the proper results.
Here's what my views filter code looks like: hidden link
If you want to see how the actual filter behaves, it's here, though you won't be able to reproduce the bug because I removed all my regions with &'s in the names.