Had a set (4) of Bach Bb slides for my C. I bought them when I mainly played C and thought it would be a cheap bay to have a Bb Strad too. They don't work very well. You have the same C trumpet intonation issues, plus a few more, with a very bright sound. I sold them on eBay a month or two ago after letting them sit in the closet for 15 years. At least they appreciated in value!
If you're talking about the Bb-style tuning slide for the C, it's supposed to help intonation. Don't have any personal experience with it, though.