I suppose there's some safety in numbers, and some value in clearly unincentivised reviews written by longstanding, respected members who are clearly trusted within the context in which they're making their comments, but by the time you try to take into account (on one side) the shills, paid reviewers, promoters and affiliates, and (on the other) the trolls, the smackheads and the people who have probably misused the product/service and/or should obviously never have bought it in the first place, it can be really difficult to form any coherent impression of trading-related products/services.
Call me old-fashioned, but I always think you have the odds slightly more in your favour, with a textbook published by a respected, mainstream, orthodox, accredited publisher (rather than just "online" where there's really no quality control at all and "anyone can publish anything"), because at least that way you know that there's been some proper "peer appraisal" and some professional quality control, before the product's released.