My limited experience says that it can be very helpful as adjunct to say MBBs for neck/low back pain. , as well as headache.
Ref:
Gobel H, et al. Botulinum toxin A in the treatment of headache syndromes and pericranial pain syndromes. Pain. 2001, 91:195-199.
Sheehan G. Botulinum toxin for the treatment of musculoskeletal pain and spasm. Curr Pain Headache Rep. 2002; 6:460-469.
Foster L, et al. Botulinum toxin A and chronic low back pain: a randomized, double-blinded study. Neurology. 2001;56; 1290-1293.
Wheeler AH, et al. Botulinum toxin A for the treatment of chornic low back pain. Pain 2001; 94:225-260.
Cheshire WP, et al. Botulinum toxin in the treatment of myofascial pain syndrome. Pain. 1994;59:65-69.
Wheeler AH, et al. A randomized, double-blind, prospective pilot study on botulinum toxin injection for refractory, unilateral, cervicothoracic, paraspinal, myofascial pain syndrome. Spine. 1998; 23; 1662-1666.
Ferrante FM, et al. Botulinum toxin type A in the treatment of myofascial pain. ANesthesiology. 2002;97;A439.