[Radek]

The Mavin is $#@&. Yes, it cannot be enchanted. Yes, there are plenty weapons around that are much better than the Mavin. Yes, it is heavy and it consumes a lot of space in your inventory. Nevertheless, only the Mavin and/or the Staff of Power (which you get later and which is also nothing to write home about) can hit Cet. Nothing else can hit him, including spells.
Therefore, keep the Mavin. If you have "mules" in the inn then pass it to them for now.
A note about a nasty trap related to the Mavin: You will fight Cet at the end of the game, in the Pyramid. It's there where you will initiate the Mavin in the Black Fire. You must do it before you enter Cet's chamber! Once you enter the chamber, the door behind you will lock and you won't be able to initiate the sword. The game is lost because only initiated Mavin (or Staff of Power) is useful against Cet.
Another note: Mavin can be used by anybody, including thieves, wizards or warlocks. If you do not have a suitable wepaon for them then give them the Mavin. It's still better than Witch's Stick.