I started doing it this year as I now live near a donor centre (previously gave whole blood at a mobile donor session). It takes longer (usually just over an hour actually rigged up to the machine), but you can eat and drink, read, listen to music or just watch TV. The machine takes blood out a bit at a time, spins it in a centrifuge to separate out the platelets, then gives the rest back to you – it’s a bit of an odd sensation when the blood is returned into your arm but you soon get used to it. The staff are really good and constantly check how you’re doing – if you find it uncomfortable they can adjust the flow rate or stop the session if necessary.
As for how you’ll feel – for me, it’s much better than after giving whole blood. I (gently) cycle home afterwards with no problems and can ride or run as hard as usual in the following days (which I couldn’t do when giving whole blood).