The symptoms of hypoxia start with a shortness of breath on exertion and headache. Restlessness, trembling, a pounding heart, weakness, malaise, blue lips, nausea, giddiness, difficulty in comprehension and judgment, and sometimes even euphoria or delirium are further signs.
Altitude is not an accurate predictor. For some, difficulties from moderate exertion can begin at 5,000 feet, for others not before 14,000 feet. Physical condition, rate of ascent, and exertion level can all have a big impact on the symptoms beyond altitude alone.
A little change in altitude can have a big impact on the riding of those not acclimatised. A rider from the coast who takes their bike to Colorado will be badly out of breath with the first pedal stroke, and will have to restrict themselves to lower gears and slower speeds.
The symptoms will gradually subside in a few days, but takes two weeks or so of living and riding to become adjusted to altitude. Note that occasionally symptoms do not subside in the normal interval, thus requiring oxygen and a descent. Try to bully your way up hills in thin air before then, and altitude sickness will bash you with headaches and nausea.
Don't expect to set any speed records when you return to sea level, unless you're in poor shape or a smoker or something. Acclimatising to altitude simply increases the lungs' ability to take in oxygen, while the usual bottleneck is the ability of the muscle cells to take in and use that oxygen. Muscle cells remains oblivious to altitude training.
Altitude sickness can be exacerbated by narcotics such as alcohol, morphine, barbituates, and by almost all anesthetic agents. Indeed, these can cause the condition at sea level.