Now that my desktop is switched to Cantarell, I'm noticing various issues in how it presents itself, often in combination of bold fonts. I'm not a designer, so take this with a grain of salt, but it, IMO, looks worse than using something like Droid Sans for the desktop.

A description of the various screenshots:

cantarell-1.png

Here, while the shell titlebar looks fine as it is, once you move to the smaller font in the window, I see problems: the 'e' and the 'a' is squashed, the bolded '6' is bad.

cantarell-2.png

The tilde character here appears disproportionately higher than the rest of the characters. (It's even more noticeable if the directory in question doesn't have a capital letter.)

cantarell-3.png

Here, on a web page, the bolded 'Calendar' does not appear to mesh well with the non bold text, such as 'Gmail' or 'Documents' next to it. Similarly, the number spacing and display as show in '9am' and '10am', doesn't mesh well with the bolded calendar dates.

My amateur opinion is that the bold adds too much width to the font, making it less of a match for the regular weight.