My suspicion is that you will see very little real traffic from blog comments other than from spiders and bots. This can be very deceiving. I don't see where very many people will click on a comment link unless the comment is exceptionally good. That is not likely to happen with a blog commenting service.
More than 95% of the comments placed on my blogs get flagged as spam, which should give you some idea about the quality you can expect from most blog commenting services. Most comment spammers are in China, Russia and Ukraine. It doesn't matter if the company you use is based in the USA if they farm the business out to those countries.
Make sure that any comments you contract for are written and submitted by living, breathing, English-speaking humans and not spam bots. You should also receive a list of all of the URLs where they placed comments. That should allow you to quickly identify the level of quality. If the comments are getting deleted or are off-topic, the service is crap.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin