It's effective if done without spamming. Commenting is essentially like handing out an invitation. If done right, with proper targeting, and with a valuable message, then blog commenting will succeed in bringing you more traffic and help you get a higher PR.
Giving a intelligent comment in blog would be a key to gain respect. If you gain respect other audiences would give attention to the product that you are advertising.
Its a great way to improve awareness of the site as well as increase the referral traffic for the site
very effective but keep in mind that only used high PR blog or related to keyword then you will see the results