Using SASS and Respond-To (Breakpoint) produces a .css file with multiple media queries, not merged.
Not a big deal, but in IE8, using css3-mediaqueries.js, cause IE
Generally, multiple media queries is not a big deal thanks to GZIP being used to compress CSS when passed from server to client.
To enable media queries support in IE7 and 8, i've been succesfully using Respond.js.
See this small guide how to combine Respond.js with Selectivizr: https://stackoverflow.com/a/16732064/901944