Grah Maitri dosha is not good. Grah Maitri dosha is formed when the rashi lords of both horoscopes are inimical or hostile to each other. In a relationship it is seen that both people when Grah Maitri dosha is there will have strong egos and will have arguments and conflicts and will be unwilling to compromise or make adjustments, as there are totally different likes and dislikes. Every relation will have conflicts that is natural, but what is not good in a relationship is to have ego clashes and partners become rigid to their stand and unwilling to make a compromise, because at the end of the day marriage is all about understanding and adjustments and walking on a joined path in life. Having said that if other factors in the horoscope matching are strong then marriage can be considered despite Grah Maitri dosha. You should get a detailed Grah Milan done for yourself and your partner to know the strength of this relationship.
Read the link below for more information on Guna and Grah milan.