The peer reviewed papers only show topical cannabinoid preparations being effective in certain neuropathic pain, such as herpetic neuralgia.
There is potential to aid in psoriasis http://onlinelibrary.wiley.com/doi/10.1111/j.1468-3083.2004.01184.x/abstract
Any other claims such as topicals being effective in decreasing arthritic pain, headaches, and muscle aches is unsubstantiated by research and based only on patient anecdotes – of which there is concern for a large bias of the placebo effect and effect of other ingredients in the products. These issues leave us unsure of the efficacy for other conditions. Research needed.
Here is the paper showing effectiveness in reducing post herpetic neuralgia: