Directory listings are a good strong backlink foundation. Niche directories should give you more targeted traffic. Regardless of submitting to free or paid directories they need to be quality.
There are a number of metrics I use to determine quality, each having it's own priority, but right up there is the directory needs to show it has consistent editorial standards.
With regards to the worth of submitting to a paid directory, look at it from a different angle. "Is your website worth the money to invest a submission fee on ?."