I use Sound Solution. I've read about recent studies that find Glucosamine Sulfate (the main ingredient in Sound Solution and some others on the market) is more effective in reaching and lubricating the joint than some of the other products containing Glucosamine Hydrochloride (has to do with the molecular structure--I have no clue 
). I've been really pleased with the results so far, but there seems to be quite a bit of debate as to what really works. I think you pretty much have to try something and see the results in your own horse--what works for one may not work for another.
Here is some information on SoundSolution:
Sound Solution - Gallon
This unique Physician and PH.D. formulated product provides visible results, GUARANTEED! Consists of 99% pure Glucosamine Sulfate - 5,000 mg/oz (to stimulate joint lubricating synovial fluid), MSM (2000mg per oz), Ascorbic Acid and L-Methionine (antioxidants), Manganese Ascorbate (for prod. of GAGs and proteoglycan cartilage lubrication), and Copper Gluconate (deficiencies linked with arthritis).
Directions:
For initial loading period (first 5 days) add 2 oz daily (based on 1,100 lbs body weight). Thereafter 1 oz daily for maintenance. Shake well before using. Protect from freezing. Store tightly closed in a cool, dry place.
Ingredients:
L-Ascorbyl-2-Polyphosphate, Boswella extract, Manganese Ascorbate, Hyaluronic Acid, Enzyme blend, Copper Gluconate, Methylsulfonylmethane, (MSM), L-Methionine, Glucosamine Sulfate, Horsetail extract, Purified water, Natural flavors, Natural form of amino acids, Proline, Glutamine, Glutamic acid.
Guaranteed Analysis:
Per 1 oz.:
Crude protein-min - 6%
Crude fat-min. - 0%
Crude fiber-min - 0%
L-Methionine - min 6%
Glucosamine Sulfate - 5000 mg
Ascorbic acid (Vitamin C) - 1063 mg
Manganese Ascorbate - 80mg
Hyaluronic Acid - 110 mg
Copper Gluconate - 6.4 mcg
MSM (methylsulfonylmethane) - 2000 mg.
