Your question is a bit vague and odd, and not likely to elicit the information you're looking for.
I've had no "problems" with my '04 Prius now in my posession for almost 6 months and with 6300+ miles...assuming you mean break downs, malfunctions, etc. There have been scattered reports of one person with a bad battery that's being replaced, a few people with ECU crashes (usually when the Service Campaign programming was being done), but that's about all I've heard about in the way of "breakdown type problems".
If by problems you mean things not performing as hoped or anticipated, then there are lots of people reporting that....the darn gas guage is a pain--you can look anywhere and find a million threads. In short, it's not linear, the final bar starts flashing with somewhere between 1/3 and 1/4 tank of fuel left (meaning around 150 miles of driving), but no way to tell exactly how low.
The driver's seat is a little uncomfortable...to me. People have complained about the inside rear-view mirror, the view out the back, the small wheels, the 'squirrelly handling' at freeway speed, not getting EPA mileage, lack of splash guards, lack of availability of the cars, etc....
What to watch for---other cars when you're driving. This car works great. Might you get a lemon, sure, but the chances of that are no greater than with any other car, and probably are lower.