I've been stuck by those many, many times over the last 15+ years. I had one in my tank that my cousin put in while he was taking care of my tank when I was in South America for 6+ months. When I got back, I took a pretty nasty hit from that one. I forgot it was in my tank and reached down to pick up a coral off the sand, after it must have knocked it off a rock, when, BAM!!!!! I got nailed by at least 15 spines, probably closer to 20. I got hit hard because I completely forgot it was in there so I reached down to pick up the coral without any caution, so it was with a good amount of force. The pain and initial feeling was very similar to a bee sting, but felt much worse because it was like getting stung by 20 bees at the same time. My hand felt like it was on fire, but after about 10-15 minutes the burning sensation was gone, but my hand still pretty tender and swollen. The swelling lasted for about an hour. You shouldn't need to go to the hospital unless you have some sort of allergic reaction to their venom, just like bees & wasps. You definitely won't get a two foot spine stuck in your arm, though. The spines can get pretty long, but a 2' spine is longer than any I've ever seen (I've definitely seen some with spines longer than 1', I've just never seen a 2' spine and don't doubt that the spines could potentially reach 2'), and the spines are extremely brittle, so I can't see any way that two feet of spine would be able to get stuck in your arm. The spine that hits your arm might be two feet long, but they are so brittle that I highly doubt that you could possibly have anything more than a 1" (one inch) tip of the spine stuck in you and even that would probably be pretty rare. The spine is far stronger closer to the carapace of the urchin, so getting it to break off so close to the carapace and not breaking off in the far more brittle tip area of the spine is virtually impossible (I only say it's virtually impossible because crazier things have happened before, but to have a 2 foot spine from a diadema urchin get stuck in your arm without breaking off at the extremely brittle tip of the spine or anywhere else is far more unlikely than winning the PowerBall when it's above $300 million. The pressure from the strike would have to be so perfectly balanced that it's a virtual impossibility in real world conditions.) You might get tiny pieces of the tip of the spine stuck in you that are splinter-sized. I had several of those tiny splinter-sized tips in my hand, but some tweezers took most of them out and the ones that the tweezers couldn't take care of came out over time as my skin cells died, regrew, etc. One thing I found pretty interesting is that the spines leave a small black ink-looking spot where they pierced my skin, kind of like tiny black freckle-sized tattoos. It probably took around 2 months for all of those little black spots to disappear. In my opinion, black long-spine urchins aren't a problem or nuisance other than knocking corals off of rocks. It stinging me was my own fault, so I don't consider that the urchins' problem, I considered it my own stupidity. I have a fondness for diadema urchins for two main reasons. In college, more than 10 years ago, I had one in my tank that I believe was crucial in protecting a few baby Banggai Cardinals. I never even knew that my two Banggais reproduced until I saw three tiny babies living between the diademas spines. Without the protection of the urchin, those 3 cardinals most likely wouldn't have survived from the appetites of other fish. Since then, I've had a lot more affection for long-spine urchins. The other reason I really appreciate diademas is because how important they are to Caribbean reefs. The huge die-off of diademas that occurred in the 80's due to a pathogen has been the biggest factor (in my opinion at least) in the decline in Florida's, and the Caribbean's, coral reefs. They were very important in consuming algae that has plagued our coral reefs for the last 3 decades.
In the end, I think they are great for our reef aquariums, as long as your corals are firmly attached to the rock and you're smart enough to remember that it's in your tank and to be cautious whenever you stick your hands in your tank. Unfortunately, I don't meet the "smart enough to remember that it's in your tank" requirement, so I don't have one in my tank right now :facepalm:.
