They are correct. Webfonts will not look the same cross platform as they use different rendering engines. I think once Windows 8 rolls out (this Friday I believe) we will see an improvement with DirectWrite.
@dida Do you have a license to use Tahoma with @font-face? As far as I’m aware, you cannot. And just because it may be preinstalled on your computer does not allow you to do so. Always check the license information.
@dida Like I stated above, you cannot use Tahoma on your website with @font-face since it is against the EULA of the desktop version. Meaning, you are not allowed to take your desktop version of Tahoma and convert it into webfont files. You have to purchase the webfont version of Tahoma which gives you a valid license.
If you’re having issues seeing the font on your machine, you can download the font for Windows. On Macs, people have posted issues with it being disabled which you can fix by a simple google search.