I would have to agree, and disagree with a mentor. It really depends on your situation. I think if you're lucky enough to be living in the US, Biggerpockets gives you more than enough information and support to get you started without a mentor, and is absolutely not necessary to be successful.
As a Canadian though, I've always looked at US real estate as a golden opportunity that i simply cannot access because i live north of the border. Biggerpockets has given me so much in terms of knowledge, and what to look for (Most importantly what to avoid) but it only goes so far since i cannot build teams in US markets alone. This is where the right mentor was crucial to my families success investing in US real estate.
I was lucky enough to get in contact with my current mentor and friend, who shared all her US contacts with my family to renovate and sell properties, giving me wholesalers to buy from, as well as helping me analyze deals. Her mentor fee was maybe ~$1.5k every 6 months, and a finders fee of $1k for every property I bought through her contacts. Needless to say, with her help, not only were we able to successfully buy and sell US properties, but we are in the position to be following her teachings and help others the same way she helped us. Soon we will be able to work alongside her, and scale into new markets as a collective group, getting more done and helping a larger number of investors compared to doing it ourselves.
It's because of my experience with my mentor that I would say finding the right one is always a great idea, but is not necessary to be successful. What is necessary though, is building and strengthening relationships with investors and people i work with in the real estate community that has truly helped my business grow; taking care of everyone involved with my investments and experience while investing in the US. As a Canadian's point of view, i deal with people first, and properties second.