The needle stick will hurt on some level, which will depend on your pain tolerance. The donation process itself should not hurt.
What are the risk factors?
You might become dizzy during or after the procedure. Some people might faint, and it's important to sit for a little while after the procedure to make sure you're ready to stand. Also, it's important to have a good meal and drink plenty of nonalcoholic fluids beforehand.
You might feel nauseated afterward, and some people vomit. Again, eating a good meal beforehand can help with this.
What's the recovery time?
After the donation, you should avoid heavy lifting or strenuous exercise. You should keep the bandage on your arm for two hours to keep the blood clot that has formed from breaking. Up to half of your platelets can generally be lost without ill effects. Your body will replace your platelets usually within 48 hours.
What's the follow-up?
You can donate platelets generally every seven days, in part because donated platelets have a shelf life of about five days.
You shouldn't have to seek any follow-up medical care after donating platelets.
Source: Cynthia Houck, supervisor and tech at the Oklahoma Blood Institute; The National Institutes of Health Clinical Center; The Mayo Clinic; the United Kingdom's National Health Service; The American Red Cross.