You only get bad memories or nightmares if you have a bad trip while on mushrooms. You can avoid a bad trip by being inside with your friends or being at any place where you are comfortable. Also, MAKE SURE YOU HAVE A FULL STOMACH BEFORE YOU DO MUSHROOMS!!!!!!! If you don't you will have a bad trip.
I don't know how much truth there is behind a full stomach or not but just be in a comfortable enviroment and always do it with a friend. Getting paranoid or nervous will more than likely perpetuate a bad trip, so don't be nervous going into it. It's nice that it starts out slowly and intensifies gradually so you're not just bombarded with a full on trip.
In all actuality there's better things you can spend your money on but if you feel the need to try it I say go for it. It's not the most horrible drug you can do but it can be a little intense at times.
By the way your hallucinations will never be as intense that you will think you're in another realm and walk out into traffic. You won't see the devil and you won't be confused to do something harmful to yourself. You're mind remains pretty clear you'll just feel different and everything feels much more significant (describing tripping is not an easy task.) At most you'll see some colors or you'll see inanimate objects sway and move, especially carpet. I always have the same hallucination at my friends house where I stare at his Jimi Hendrix poster and his hair begins to move and grow a little bit.