Blog commenting can be a great way to drive more traffic to your own site and gain you some points with the search engines. A few more tips I would add is to seek out blogs and sites that are highly relavent to your niche, your target market, or a specific service you provide - that way the audience who comes into contact with your posts will be able to see that you're an expert in your field, that you have something valuable to contribute, and that you take the time to add that expertise to the great community.
When you get your voice out there it's great for
SEO, but can also be wonderful for building a reputation with human readers as well!