Make sure you get a real HTC U250. HTC might send you a replacement if you call them and ask sweetly. Anything that is a proper AC charger, 5W or more, 1A (1000mA) or more should do. I found -- perplexing though it is -- that the supplied HTC cable. T500, charges better than other no-name cables.
While you're charging, go to Settings | Power ... it should say "Charging (AC)." If is says otherwise, e.g. "Charging (USB)" while plugged into a wall charger, the charger isn't a proper fast-charge charger.