I recently purchased a Percheron x Paint - only a half Percheron, but I can give my two cents. My guy has a super temperament - very friendly and such - and is very quiet, even at 3 years. Doesn't spook at much.
However, he is less sensitive than say a thoroughbred. He requires a bit more motivation, but that's okay, because it's easier to teach a horse to go than to stop and a much less dangerous feat. My guy is really comfortable, has smooth, fluid, ground-covering gaits, and has a soft mouth - he's very bendy. They CAN jump, no matter what people will tell you, but they aren't the athletic showjumpers that you'll be taking into competitions of course. They also aren't really suited for certain types of showing unless you're doing unrated shows or draft undersaddle shows - for instance, they wouldn't excell in hunter/jumper classes. They are often seen in dressage, field hunting, eventing, foxhunting, etc. They also make good trail horses.
As far as the bad - if you look up general problems draft horses have, that would apply to the Percheron breed. A lot of draft horses require oil in their feed. If you're going to be using him as a riding horse, I'd probably look into a feeding regimen that might add a bit more energy, because they have naturally calm dispositions. They are working horses though, so they are used to going for long periods, but they aren't used to going for long periods intensely - that's the difference. They are bred to work long and slow.
The main issues I'm having with my guy are on the ground. They are bigger and less sensitive, so they generally require you to be more firm when handling them. They can get pushy and walk on your toes and stuff, not in a menacing way, but just because they are more dull and therefore become more disrespectful of wee little humans. I'm trying the Clinton Anderson Ground Control & Respect series to fix that area with my gelding.