How good is light therapy for helping with insomnia?

Not very likely. Normally light therapy is used for circadian issues or depression. Insomnia responds better to cognitive behavioral therapy.
Poor. Light therapy is not a good treatment for insomnia. It is typically used in circacadian rhythm disturbances (ie delayed or advanced phase sleep syndrome) but not in insomnia. Please do not try using light therapy without the supervision of a sleep physician because you can greatly affect/harm your sleep/wake cycle.

Related Questions

I have mild/moderate acne, using blue/red light therapy mask for it, its helping, but insomnia has worsened, does the light block my own melatonin?

Morning only. You shouldn't be doing that therapy at night as it may interfere with your sleep. Read more...
See a dermatologist. You are already taking melatonin. You may need to relax better at night. You will probably obtain best result for acne with antiandrogen spironolactone 100mg daily. Costs $16 for 3 months, very safe, much more effective. If satisfied after 3 months, continue as long as needed. There are simple relaxation techniques which help many people to sleep. Read more...

Do light therapy boxes work for people with insomnia? Are there any side effects?

May. Be effective for some people. Especially in those who have little exposure to light through out the day. Read more...
Light therapy. Light therapy has been used to treat: Jet lag, transitions to night work schedules, shift work related sleep disturbances, Delayed Sleep Phase Syndrome (DSPS), Advanced Sleep Phase Syndrome (ASPS), non-24-hour sleep-wake syndrome & age-related sleep maintenance insomnia. For risks of light therapy see: http://www.mayoclinic.org/tests-procedures/light-therapy/details/risks/cmc-20197423. Read more...

Using an over-the-counter blue/red light therapy mask for acne, 15 min daily, my insomnia has worsened, it is possible its the light therapy?

Poor choice. Phototherapy is at best an adjunct for acne. You deserve better. If topical benzoyl peroxide as tolerated has not cleared you in two weeks, get with your personal physician who can add prescription-strength therapy. Acne is a serious chronic illness that needs to be treated aggressively; having visible acne as an adult is a sign of poor self-care. Wishing you the best. Read more...