They tend to smell a lot when you turn them back on - best to paint them when you turn off the heating for the summer so they have several months before they get heated again.
What colour? If white, beware of ordinary oil-based paints - they may turn yellow quite quickly. There are any number of paints available specifically for radiators. Google for 'gloss radiator paint' and take your pick.
The pitfalls - smell, discolouring - of using gloss have already been pointed out. There is such a thing as radiator paint but there is also Satin Finish gloss which barely smells and does not quickly discolour. Again, paint whilst radiators are cold, preferably a good few hours before they are switched back on - maybe before bed.
I usually use the aerosol cans of radiator paint.
Dead easy - sand down and apply a few light coats, leaving it approx 15-30 mins between each coat. no need for a paint brush, just spray a coat on, leave to dry and re-apply another coat if needed. No runs and looks great :)
Most of my customers want the radiator to match the wall colour, so using radiator paint is out of the question. I usually use oil-based eggshell or satinwood in the wall colour after I've de-greased it with meths & given it a light rub-down. I also paint the radiator whilst its still warm...the paint flows better & it's easier to remove the runs from the intricate parts.