Have you found these link building services beneficial? I am looking into link building services and I wonder if they are worth it.

I wouldn't really go for that. There's no guarantee for long term links.

I am very skeptical of anyone offering to "build" links. I find it best to earn citations rather than fabricate them.

For link building purpose, I would rather trust tools like IBP or SEO Elite but not those services. BTW, I usually build backlinks manually via social bookmarking, forum posting and blog commenting.