Nd:YAG laser using a carbon-based lotion was promoted as long-term and even permanent for years, until published clinical data showed that this was not true.  The best overview of Nd:YAG history, benefits and limitations is Littler’s 1999 article. 
Q-switched with carbon-based lotion
Early published medical papers using Nd:YAG with the carbon lotion reported from 27% to 66% reduction at 3 months after one treatment.  One 15-subject study found long pulse Nd:YAG more effective and less painful when compared to alexandrite treatment 
One large study of patient satisfaction at six months after multiple treatments showed those seeking to thin hair were generally pleased, but 75% of those expecting more were not pleased.  One study observed full regrowth by month six . Side effects were observed in one study to be less likely with this treatment compared to other lasers.  Another also saw fewer side effects than ruby or alexandrite 
One study confirms observation that treatment is probably more effective on anagen hairs.  Histological evidence suggests thermal damage occurs to about 1 mm deep. 
One small study suggests promise for treatment of “shaving bumps” (pseudofolliculitis barbae)  and another reports on successful treatment of infants born with excessive amounts of downy hair (hypertrichosis lanuginosa congenita) 
Long pulse Nd:YAG
The one published study found 15 subjects had 59% reduction at 3 months 
An unpublished report observed skin discoloration in tanned skin is rare and resolved by 6 months. Eleven patients followed had about 30% reduction at 6 months.  Another unpublished manufacturer report  with a long-term follow up  observed that 25 subjects had median of 27% reduction at 3 months after a single treatment. Also in this report, observers of before and after photos thought the after photos looked better in three-fourths of the treated areas, with the control looking better in half the treated areas. Sites receiving one or two treatments were observed to have 39% and 50% reduction at 12 months, respectively.
Please see hairfacts.com for a list of Nd: YAG laser medical data.
Littler CM. Hair removal using the Nd:YAG laser system. Dermatologic Clinics 1999 Apr;17(2):401-30, x.
Goldberg DJ, Littler CM, Wheeland RG. Topical suspension-assisted Q-switched Nd:YAG laser hair removal. Dermatologic Surgery 1997 Sep;23(9):741-5.
Rogers CJ, Glaser DA, Siegfried EC, Walsh PM. Hair removal using topical suspension-assisted Q-switched Nd:YAG and long-pulsed alexandrite lasers: A comparative study. Dermatologic Surgery 1999 Nov;25(11):844-47; discussion 848-50.
Bencini PL, Luci A, Galimberti M, Ferranti G. Long-term epilation with long-pulsed neodimium:YAG laser. Dermatologic Surgery1999 Mar;25(3):175-8.
Nanni CA, Alster TS. Optimizing treatment parameters for hair removal using a topical carbon-based solution and 1064-nm Q-switched neodymium:YAG laser energy. Archives of Dermatology 1997 Dec;133(12):1546-9.
Nanni CA, Alster TS. Laser-assisted hair removal: side effects of Q-switched Nd:YAG, long-pulsed ruby, and alexandrite lasers. Journal of the American Academy of Dermatology. 1999 Aug;41(2 Pt 1):165-71.
Kolinko VG, Littler CM, Cole A. Influence of the anagen:telogen ratio on Q-switched Nd:YAG laser hair removal efficacy. Lasers in Surgery and Medicine 2000;26(1):33-40.
Goldberg DJ, Silapunt S. Histologic evaluation of a millisecond Nd:YAG laser for hair removal. Lasers in Surgery and Medicine 2001;28(2):159-61.
Rogers CJ, Glaser DA. Treatment of pseudofolliculitis barbae using the Q-switched Nd:YAG laser with topical carbon suspension. Dermatologic Surgery 2000 Aug;26(8):737-42.
Littler CM. Laser hair removal in a patient with hypertrichosis lanuginosa congenita. Dermatologic Surgery 1997 Aug;23(8):705-7.
Goldberg DJ, Samady JA. Evaluation of a long-pulse Q-switched Nd:YAG laser for hair removal. Dermatologic Surgery 2000 Feb;26(2):109-13.
Grossman M. Hair Removal for Tanned Patients & Wide Range of Skin Types with the Altus Nd:YAG CoolGlide Laser – Study Results (unpublished 11/2000)
Kilmer SL. 3 Month Clinical Results Using the CoolGlide Long-Pulse Nd:YAG Laser for Hair Removal. (unpublished and undated)
Kilmer SL. Summary of 15 Month Clinical Results Using the CoolGlide Long-Pulse Nd:YAG Laser for Hair Removal. (unpublished, June, 2000)
Summary of data with links to abstracts
Human clinical trials
Finkelstein (1990, 1991): reports on 4 patients requiring urethral grafts. To avoid hair on the grafts, the grafts were treated with Nd:YAG laser. This early use of Nd:YAG differed from current cosmetci hair removal applications, since scarring was not an issue.
Kuriloff (1988): Reviews laser epilation for hair growth in the esophagus. This early use of Nd:YAG differed from current cosmetic hair removal applications, since scarring was not an issue.
Goldberg (1997): 35 subjects received one Nd:YAG treatment. At 12 weeks, patients averaged 44-66% hair reduction.
Nanni (1997): 12 patients were treated once on 18 areas. Each area was divided in 4 and treated with separate protocols for each quadrant: waxing alone, laser alone, waxing, followed by laser, and waxing followed by a carbon lotion and laser. Mean percentage of hair regrowth at 1 month was 39.9% for the wax-carbon-laser quadrant, 46.7% for the wax-laser quadrant, 66.1% for the laser-alone quadrant, and 77.9% for the wax control quadrant. The percentage of hair regrowth approximately doubled by 3 months but was significantly delayed in all laser-treated quadrants regardless of pretreatment protocol. Full hair regrowth in all anatomic locations was observed by month 6.
Littler (1997): Discusses use of Nd:YAG in the treatment of excessive downy hair, a hereditary condition.
Littler (1999): This is not only the best review of Nd:YAG literature, it is also one of the most comprehensive laser articles, period. Superbly researched and written.
Bencini (1999): 208 subjects received 1-8 monthly treatments followed by 1-6 months of observation. In the group with normal hair distribution, only one in four were pleased with the result of one treatment at six months. Those just seeking to thin hair were pleased at six months. Among those who required complete epilation, it took 4-6 monthly treatments to reach that state, and results at follow-ups are not discussed. Facial hair took the most number of treatments.
Rogers (1999): 15 subjects had 2 treatments on one armpit. 3 months after treatment average reduction was 27%.
Grevelink (1996): Five Black patients had successful tattoo removal, and longer pulsewidths are recommended.
Anderson (1993): Five patients saw immediate and permanent darkening of white, pink, or flesh-colored tattoos, two of which required surgical removal.
Kilmer (1993): 25 patients with 39 tattoos were treated, some after failed ruby laser treatment. Black ink came out well (28% saw almsot complete reduction of black), but other colors did not respond as well.
Kilmer (1997): A good review of all Q-switched lasers used for tattoo removal.
Landthaler (1986): Four patients with pigmented lesions responded well to Nd:YAG, but had more scarring compared to other lasers, possibly due to longer exposures and no contact cooling.
Kilmer (1994): 49 patients treated for benign pigmented lesions saw varying improvement in this well-designed study.
Tse (1994): 20 patients received both Q-switched Nd:YAG and ruby laser for pigmented lesions and say a minimum 30% improvement with one treatment.
Grossman (1995): 9 cafe au lait macules were treated with an Nd:YAG and a ruby laser. As with hair removal, response to laser was variable and unpredicatable.