I have a seahorse in a reef tank and he is doing fine. I have to agree with the things that are said, yes, they are a little more on the difficult side, slow moving, so other fish tend to 'beat' them to the food, etc. But if you do a lot of reasearch on seahorses, and keep all the 'drawbacks' in mind when purchasing one, you should be ok. I had been told to never keep a seahorse in a reef tank becuase he won't eat, etc, but I got one with the intention of feeding my other fish first, then feeding him with a baster after. In my case, i was surprised to find out that even though i have offered him plenty of food- he has of yet to eat food offered. this worried me at first, but then after sitting and watching him one afternoon for literally 2 hours, i noticed that he is actually eating stuff from my live rocks. as for the current, we have some areas that are lower current, and i figured that he would just stay in those areas, but we have been pleasantly surprised to see that he actually likes to 'ride' the current--- its really funny. A couple of times, we have seen him make his way to apowerhead, hook on and position his body so that he is RIGHT in the flow, and then let go and 'surf'!
So-- if you plan on getting a seahorse, do LOTS of research, make sure your watr conditions are pristine, and make sure you have the time to invest in making sure he does well. I would also recomend that you try to find one that is tank raised- they are more likely to eat frozen food instead of live food, and this is also good for the declining wild seahorse population.
Check out my cam-- occaisonally my seahorse will make an appearance!