Everyone knows blog commenting can be a good way to get some traffic from other blogs. However there's no guarantee that you'll get any traffic from blogs which get lots of traffic because odds are if you leave a comment and it's not on the first page, it might not be read. Your hard work will all have been for naught. If you know a comment you'll write won't be on the blog's first page then I think it would be better to post comments on sites that don't get much traffic. Odds are people who read your comment [if it's helpful] may want to find out more about you, click on your link and visit your site. So don't write off low traffic sites. They too, can be good traffic sources for you. All the best!