Has been discussed on these forums numerous times (usually on the Soils, Compost and Mulch forum) with the consensus being it is a good, easily/freely obtainable but slow to decompose source of organic matter. If not willing to wait for it to decompose in your compost pile, you could certainly use it as a mulch, although I'd be inclined to rinse it thoroughly to get rid of any salt. In fact, salt build up or concentrations may be why it takes so long to decompose.....perhaps the soil biology is not fond of their food being salted so they avoid :-) Just pure speculation on that aspect.............. :-))

You're right gardengal I probably need to be more patient , but whenever I have composted it I did give a very thorough rinse to remove as much salt as possible and it still lasted a long time. Would like to try kelp but that rarely washes up on the beaches closer to me. Have you ever tried using fresh kelp for your gardens?