The update to v2.6 works from with VS 2012 and I have the Web Tools 2012.2 update installed, but a problem that I noticed that does not exist in the v2.5.1 version (I downgraded to fix for now) is that the @import lines in LESS files do not locate the referenced files anymore when compiling.
The compiler bombs unless the file referenced in the @import line is in the same physical folder. If any path information exists, such as "../../style/common.less", the compiler responds with an error and won't create the generated CSS file.
The error that I get is that it cannot find the file and the path information is truncated to either the first parent path construct or none. So, it's looking for "../style/common.less" or "style/common.less". It seems to be intermittent between the two.
For a possibly related issue, see: LESS importing CSS and relative paths
UPDATE: Confirmed that the expected behavior works correctly in LESS v1.3.1 and below.
I updated to v2.6 of Web Essentials via the Visual Studio extensions update dialog. Then I replaced the contents of the LESS script file in the path X:\Users\{USER}\AppData\Local\Microsoft\VisualStudio\11.0\Extensions\{HASH}\Resources\Scripts\less-1.3.0.js (which was v1.3.3 originally with the Web Essentials v2.6 update) with the scripts for versions 1.3.0, 1.3.1, 1.3.2, 1.4.0-beta, 1.4.0 and 1.4.1. The compiles only worked with versions 1.3.1 and lower.
I'm wondering if something got broken when support for @import-once was removed.
UPDATE 2: Version 2.7 of Web Essentials still exhibits this behavior, but can be fixed also as per above.