It's considered bad Etiquette.
If you had a piece of content for people to read or felt necessary to mention a product/service then it's OK. But most will frown on it as spam including search engine

It's good for backlinks if you really want to rank your site.

And in the name section try gambling with using your website name, keywords and your name.

The only downside i see with commenting is, the OBL isn't controllable.

You will be better off commenting on regulated blogs than auto approve.

Leave it.

Don't expect much or anything out of it really though.

This is the job of Blackhat backlink builder. You can use Senuke.

Commenting can be considered as both White hat and Black hat. It is just like using a knife. A knife with doctor can save the patients, the same knife with a killer is dangerous for everyone around him. Knife on its own is neither bad nor good, it depends the person who is handling it. The same logic applies for blog commenting.

White hat methods of doing comments

1) Read the blog contents and leave meaningful topic related comment. Not generic spam comments like good article, thanks, quality stuff etc.

2) Use only your original name as name

3) Post the comments only in niche relevant sites.

4) Identify the niche relevant high traffic websites and leave good useful comments. If your comments are helpful, it will naturally yield you some traffic even if the link is no follow. Do not worry about the Page pr, authority etc.

Black Hat Method of doing comments

1) Use of automated tools like scrapebox, gscraper etc to spam other websites.

2) Spamming comments in irrelevant sites just because they are High PR or do follow.

3) Use of anchor texts in all the comments

4) Even if done manually in niche relevant sites, leaving one line or meaningless comments.

5) Buying / posting links in auto approve sites

Zero SEO benefits, but you will get some traffic from that. But, its not worth it unless you have a specific plan around getting traffic and making sales relying completely on comment traffic.

If you leave a comment on blogs that are relevant to your niche - it's great

There are no direct ranking benefits, but it'll help you to get the word out about your brand and maybe bring you some referral traffic.

Don't use your keywords as an anchor text and always write solid, constructive comments