Materia Magica is the best mud active in internet. But nothing is perfect, so MM has some few bad points too.
The multiclass advance system is very original ( consists in obtain some magic stones that only avaliable in very-few places). But the number of magic stones required is very high! 250 for first mult-class! It can be considered the worst point in this game. Many players that dont have patience to get these stones, stop to play the mud or only play for duels.
However, the mud is wonderful. Many skills, spells, quests. Advanced options to play in a formation. A huge world with many cool places to explore. PRETTY seasail system. High role-play!
In conclusion: Materia Magica is the best.