No see ums (because they're so small), otherwise known as sandflies or sandfleas, are probably the thing people hate most about our beautiful island. If you look through travel forums, you'll see many posts about these horrible little beasts. Are they that bad, is it a reason to run from the island in terror? Well, forewarned is forearmed. If you know they're out there, you can guard against them. In truth, many people aren't even affected by their bite, but for those that are, the itchy little bites (there's never just one, more like 20!), can be really, really itchy and can ruin an otherwise idyllic stay. The sensible strategy is to avoid being bitten!
Sandflies can fly, and they lay their eggs in the sand, so usually they're on the beach, but can travel inwards as well. The best preparation is just to always put on repellent. They tend to be worse at dusk, and they love salty skin, if you've just come out of the sea after a swim. Anything with DEET will protect against them, and a product called Cactus Juice also works well. Some people opt for Skin So Soft or baby oil (it basically leaves your skin to greasy for them to attack). Keep repellent in your bag with you, so that you're always prepared.
If you get bit, how do you know it? The bite of a no-see-um is red, with a little deeper-red pinprick size dot in the middle. Sometimes the bite is flat (generally for those who don't get an itchy reaction), and sometimes it's raised, like a mosquito bite. Normally, a person will have a ton of bites in an area, which can make for a fun game of connect the dots, if you have a pen handy. They'll bite your legs and ankles, your arms, your back. If you've gotten lots of bites, I'd suggest taking benadryl as well as applying topical hydro-cortisone cream. The bites usually last about a week. The good news is that, after a few weeks of exposure, you'll build up a tolerance to them, so if you've been dreaming about moving down, but have gotten scared off by these little critters, don't abandon your dream :)